Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2017	Oct. 31, 2017
Document And Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3
Entity Registrant Name	CarGurus, Inc.
Trading Symbol	CARG
Entity Central Index Key	0001494259
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Class A Common Stock
Document And Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		77,877,494
Class B Common Stock
Document And Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		28,213,276

Unaudited Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 25,636	$ 29,476
Investments	60,000	44,774
Accounts receivable, net of allowance for doubtful accounts of $141 and $164, respectively	10,123	6,653
Prepaid income taxes		1,815
Prepaid expenses and other current assets	3,485	2,789
Total current assets	99,244	85,507
Property and equipment, net (Note 4)	16,100	12,780
Restricted cash	1,783	2,044
Deferred tax assets	371
Other long-term assets (Note 5)	4,158
Total assets	121,656	100,331
Current liabilities
Accounts payable	22,737	16,426
Accrued expenses (Note 6)	9,953	8,384
Deferred revenue	4,598	3,330
Accrued income taxes	156
Deferred rent	1,144	910
Total current liabilities	38,588	29,050
Deferred rent, net of current portion	5,701	5,673
Deferred tax liabilities		292
Other non-current liabilities	969	590
Total liabilities	45,258	35,605
Commitments and contingencies (Note 7)
Convertible preferred stock (Note 8)	132,698	132,698
Stockholders’ deficit:
Additional paid-in capital	4,225	3,714
Accumulated deficit	(60,766)	(71,698)
Accumulated other comprehensive income (loss)	199	(30)
Total stockholders’ deficit	(56,300)	(67,972)
Total liabilities, convertible preferred stock, and stockholders’ deficit	121,656	100,331
Class A Common Stock
Stockholders’ deficit:
Common stock	14	14
Class B Common Stock
Stockholders’ deficit:
Common stock	$ 28	$ 28

Unaudited Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Allowance for doubtful accounts	$ 141	$ 164
Class A Common Stock
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	120,020,700	120,020,700
Common stock, shares issued	14,107,816	14,022,132
Common stock, shares outstanding	14,107,816	14,022,132
Class B Common Stock
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	80,013,800	80,013,800
Common stock, shares issued	28,213,276	28,044,264
Common stock, shares outstanding	28,213,276	28,044,264

Unaudited Condensed Consolidated Income Statements - USD ($)
$ in Thousands

3 Months Ended

9 Months Ended

Sep. 30, 2017

Sep. 30, 2016

Sep. 30, 2017

Sep. 30, 2016

Income Statement [Abstract]

Revenue

$ 82,989

$ 53,136

$ 226,264

$ 137,377

Cost of revenue

^[1]

4,720

2,852

12,367

6,671

Gross profit

78,269

50,284

213,897

130,706

Operating expenses:

Sales and marketing

63,891

40,510

168,495

108,823

Product, technology, and development

5,796

2,984

14,153

8,134

General and administrative

5,006

3,101

14,098

8,719

Depreciation and amortization

713

432

1,909

1,065

Total operating expenses

75,406

47,027

198,655

126,741

Income from operations

2,863

3,257

15,242

3,965

Other income, net

106

107

323

260

Income before income taxes

2,969

3,364

15,565

4,225

Provision for income taxes

590

1,226

4,633

1,566

Net income

2,379

2,138

10,932

2,659

Reconciliation of net income to net income attributable to common stockholders:

Net income

2,379

2,138

10,932

2,659

Net income attributable to participating securities

(1,401)

(1,260)

(6,446)

(1,554)

Net income attributable to common stockholders — basic

978

878

4,486

1,105

Net income attributable to participating securities

(1,345)

(1,222)

(6,198)

(1,507)

Net income attributable to common stockholders — diluted

$ 1,034

$ 916

$ 4,734

$ 1,152

Net income per share attributable to common stockholders: (Note 10)

Basic

$ 0.02

$ 0.11

$ 0.02

Diluted

$ 0.02

$ 0.10

$ 0.02

Weighted-average number of shares of common stock used in computing net income per share attributable to common stockholders:

Basic

42,262,035

44,692,419

42,168,904

44,665,063

Diluted

46,567,173

48,069,373

46,310,630

48,040,754

[1]	Includes depreciation and amortization expense for the three months ended September 30, 2017 and 2016 and for the nine months ended September 30, 2017 and 2016 of $370, $113, $761, and $316, respectively.

Unaudited Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 2,379	$ 2,138	$ 10,932	$ 2,659
Other comprehensive income:
Foreign currency translation adjustment	72	1	229	(18)
Comprehensive income	$ 2,451	$ 2,139	$ 11,161	$ 2,641

Unaudited Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating Activities
Net income	$ 10,932	$ 2,659
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	2,670	1,381
Unrealized currency loss on foreign denominated transactions	96
Deferred taxes	(663)	204
Provision for doubtful accounts	544	334
Stock-based compensation expense	224	236
Changes in operating assets and liabilities:
Accounts receivable	(4,013)	(568)
Prepaid expenses, prepaid income taxes, and other current assets	1,143	(2,838)
Accounts payable	6,409	11,485
Accrued expenses	(741)	2,153
Deferred revenue	1,265	1,705
Deferred rent	262	2,049
Accrued income taxes	156	1,190
Other non-current liabilities	258	461
Net cash provided by operating activities	18,542	20,451
Investing Activities
Purchases of property and equipment	(4,247)	(4,009)
Capitalization of website development costs	(1,487)	(913)
Investments in certificates of deposit	(50,000)	(41,774)
Maturities of certificates of deposit	34,774	5,000
Net cash used in investing activities	(20,960)	(41,696)
Financing Activities
Proceeds from issuance of preferred stock		60,000
Proceeds from exercise of unit options and stock options	288	92
Payment of deferred initial public offering costs	(2,128)
Cash paid for repurchase of preferred stock, common stock, and vested options		(1,262)
Net cash (used in) provided by financing activities	(1,840)	58,830
Impact of foreign currency on cash, cash equivalents, and restricted cash	157	(26)
Net (decrease) increase in cash, cash equivalents, and restricted cash	(4,101)	37,559
Cash, cash equivalents, and restricted cash at beginning of period	31,520	62,863
Cash, cash equivalents, and restricted cash at end of period	27,419	100,422
Supplemental disclosure of cash flow information:
Cash paid for income taxes	4,220	6
Cash paid for interest	17	20
Supplemental disclosure of non-cash investing and financing activities:
Unpaid purchases of property and equipment	739	1,774
Unpaid deferred initial public offering costs	$ 2,014
Unpaid preferred stock issuance costs		$ 268

Organization and Business Description	9 Months Ended
Organization and Business Description	Sep. 30, 2017
Accounting Policies [Abstract]
Organization and Business Description	1. Organization and Business Description CarGurus, Inc. (the “Company”), is a global, online automotive marketplace connecting buyers and sellers of new and used cars. Using proprietary technology, search algorithms, and innovative data analytics, the Company provides information and analysis that create a differentiated automotive search experience for consumers. The Company’s marketplace empowers users worldwide with unbiased third-party validation on pricing, dealer reputation, and other useful information that aids them in finding “Great Deals from Top-Rated Dealers.” The Company is headquartered in Cambridge, Massachusetts and was incorporated in the state of Delaware on June 26, 2015. The Company operates principally in the United States and has also launched marketplaces in Canada, the United Kingdom, and Germany. The Company has wholly owned subsidiaries in the United States, Ireland, and the United Kingdom. The Company is subject to a number of risks and uncertainties common to companies in similar industries and stages of development including, but not limited to, rapid technological changes, competition from substitute products and services from larger companies, management of international activities, protection of proprietary rights, patent litigation, and dependence on key individuals.

Summary of Significant Accounting Policies	9 Months Ended
Summary of Significant Accounting Policies	Sep. 30, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying interim condensed consolidated financial statements (the “Unaudited Condensed Consolidated Financial Statements”) are unaudited. These Unaudited Condensed Consolidated Financial Statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2016 included in the Company’s final prospectus related to the initial public offering (“IPO”), filed with the Securities and Exchange Commission (“SEC”) pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended, on October 12, 2017 (the “Prospectus”). The Unaudited Condensed Consolidated Financial Statements have been prepared pursuant to the rules and regulations of the SEC. Accordingly, certain information and footnote disclosures normally included in financial statements prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. The Unaudited Condensed Consolidated Financial Statements reflect all adjustments, consisting of only normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of September 30, 2017 and December 31, 2016, results of operations for the three months and nine months ended September 30, 2017 and 2016, and cash flows for the nine months ended September 30, 2017 and 2016. These interim periods are not necessarily indicative of the results to be expected for any other interim period or the full year. The accompanying Unaudited Condensed Consolidated Financial Statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the Unaudited Condensed Consolidated Financial Statements. As of September 30, 2017, there have been no material changes in the Company's significant accounting policies from those that were disclosed in the Prospectus except as discussed below. In March 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-09, Compensation — Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting (“ASU 2016-09”). The guidance identifies areas for simplification involving several aspects of accounting for share-based payments, including income tax consequences, classification of awards as either equity or liabilities, an option to make a policy election to recognize gross stock-based compensation expense with actual forfeitures recognized as they occur, as well as certain classification changes on the statement of cash flows. The Company adopted ASU 2016-09 on January 1, 2017 and elected to account for forfeitures when they occur, on a modified retrospective basis. The cumulative effect adjustment related to the Company's accounting policy change for forfeitures was not material. In accordance with the adoption of this guidance, the tax effect of differences between tax deductions related to stock compensation and the corresponding financial statement expense compensation will no longer be recorded to additional paid-in capital in the balance sheet. Instead, such amounts will be recorded to tax expense. During the three and nine months ended September 30, 2017, the Company recorded tax benefits of $267 and $640, respectively, related to differences between tax deductions related to stock compensation and the corresponding financial statement expense compensation. The Company also elected to prospectively apply the change in presentation of excess tax benefits, wherein excess tax benefits recognized on stock-based compensation expense is now classified as an operating activity in the consolidated statements of cash flows. The Company did not adjust the classifications of excess tax benefits in its condensed consolidated statements of cash flows for the nine months ended September 30, 2016. The adoption did not have any other material impact on the Company's condensed consolidated financial statements. Principles of Consolidation The accompanying Unaudited Condensed Consolidated Financial Statements include the accounts of CarGurus, Inc. and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. The Company prepares its Unaudited Condensed Consolidated Financial Statements and related disclosures in conformity with GAAP. Subsequent Event Considerations The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure, other than those disclosed in this Quarterly Report on Form 10-Q. Use of Estimates In preparing its Unaudited Condensed Consolidated Financial Statements in accordance with GAAP, the Company is required to make estimates and assumptions that affect the amounts of assets, liabilities, revenue, costs, and expenses, and disclosure of contingent assets and liabilities which are reported in the Unaudited Condensed Consolidated Financial Statements and accompanying disclosures. The accounting estimates that require the most difficult and subjective judgments include revenue recognition and revenue reserves, contingent liabilities, allowances for doubtful accounts, expected future cash flows used to evaluate the recoverability of long-lived assets, the expensing and capitalization of product, technology, and development costs for website development and internal-use software, the determination of the fair value of stock awards issued, stock-based compensation expense, and the recoverability of the Company's net deferred tax assets and related valuation allowance. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could materially differ from the Company’s estimates and assumptions. Emerging Growth Company Status The Company is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act, or JOBS Act, and may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. The Company may take advantage of these exemptions until the Company is no longer an emerging growth company. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected to use the extended transition period for complying with new or revised accounting standards and as a result of this election, its financial statements may not be comparable to companies that comply with public company effective dates. The Company may take advantage of these exemptions up until December 31, 2022 or such earlier time that it is no longer an emerging growth company. The Company would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue, it has more than $700.0 million in market value of its stock held by non-affiliates (and it has been a public company for at least 12 months, and has filed one annual report on Form 10-K), or it issues more than $1.0 billion of non-convertible debt securities over a three-year period. Concentration of Credit Risk The Company has no significant off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, investments, and trade accounts receivable. The Company maintains its cash, cash equivalents, and investments principally with accredited financial institutions of high credit standing. Although the Company deposits its cash and investments with multiple financial institutions, its deposits, at times, may exceed federally insured limits. Credit risk with respect to accounts receivable is dispersed due to the large number of customers. The Company routinely assesses the creditworthiness of its customers. The Company generally has not experienced any material losses related to receivables from individual customers, or groups of customers. The Company does not require collateral. Due to these factors, no additional credit risk beyond amounts provided for collection losses is believed by management to be probable in the Company's accounts receivable. For the nine months ended September 30, 2017 and the year ended December 31, 2016, no individual customer accounted for more than 10% of total revenue. As of September 30, 2017, three customers accounted for 17%, 15%, and 10% of net accounts receivable, respectively. As of December 31, 2016, two customers accounted for 24% and 15% of net accounts receivable, respectively. No other individual customer accounted for more than 10% of net accounts receivable at September 30, 2017 or December 31, 2016. Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company on or prior to the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which modifies how all entities recognize revenue, and consolidates into one Accounting Standards Codification (“ASC”) Topic (ASC Topic 606, Revenue from Contracts with Customers) the current guidance found in ASC Topic 605, and various other revenue accounting standards for specialized transactions and industries. ASU 2014-09 outlines a comprehensive five-step revenue recognition model based on the principle that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 may be applied using either a full retrospective approach, under which all years included in the financial statements will be presented under the revised guidance, or a modified retrospective approach, under which financial statements will be prepared under the revised guidance for the year of adoption, but not for prior years. Under the latter method, entities will recognize a cumulative catch-up adjustment to the opening balance of retained earnings at the effective date for contracts that still require performance by the entity at the date of adoption. In August 2015, the FASB issued ASU No. 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of Effective Date (“ASU 2015-14”), which defers the effective date of ASU 2014-09 by one year. ASU 2014-09 is now effective for public entities for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. The Company has developed an implementation plan to adopt this new guidance. As part of this plan, the Company is currently assessing the impact of the new guidance on its results of operations. Based on the Company’s procedures performed to date, nothing has come to its attention that would indicate that the adoption of ASU 2014-09 will have a material impact on its revenue recognition; however, further analysis is required and the Company will continue to evaluate this assessment throughout 2017 and 2018. While the Company is still evaluating the impact that this guidance will have on its financial statements and related disclosures, the Company’s preliminary assessment is that there will be an impact relating to the accounting for costs to acquire a contract. Under ASU 2015-14, the Company will be required to capitalize certain costs, primarily commission expense to sales representatives, on its consolidated balance sheet and amortize such costs over the period of performance for the underlying customer contracts. The Company is still evaluating the impact of capitalizing costs to execute a contract. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”). ASU 2016-02 requires a lessee to recognize most leases on the balance sheet but recognize expenses on the income statement in a manner similar to current practice. The update states that a lessee will recognize a lease liability for the obligation to make lease payments and a right-to-use asset for the right to use the underlying assets for the lease term. Leases will continue to be classified as either financing or operating, with classification affecting the recognition, measurement, and presentation of expenses and cash flows arising from a lease. For public entities, the new standard is effective for interim and annual periods beginning on or after January 1, 2019, with early adoption permitted. For non-public entities, the new standard is effective for annual periods beginning after December 15, 2019, with early adoption permitted. The Company is evaluating the impact this guidance may have on its consolidated financial statements. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”). ASU 2016-15 is intended to add or clarify guidance on the classification of certain cash receipts and payments in the statement of cash flows and to eliminate the diversity in practice related to such classifications. For public entities, the guidance in ASU 2016-15 is required for annual reporting periods beginning after December 15, 2017, with early adoption permitted. For non-public entities, the guidance is effective for annual reporting periods beginning after December 15, 2018, with early adoption permitted. The Company is currently in the process of evaluating the impact and timing of adoption of ASU 2016-15 on its consolidated financial statements.

Fair Value of Financial Instruments Including Cash, Cash Equivalents and Investments

9 Months Ended

Sep. 30, 2017

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments Including Cash, Cash Equivalents and Investments

3. Fair Value of Financial Instruments Including Cash, Cash Equivalents and Investments

ASC 820, Fair Value Measurements and Disclosures, establishes a three-level valuation hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances.

ASC 820 identifies fair value as the exchange price, or exit price, representing the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants based on the highest and best use of the asset or liability. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. The Company uses valuation techniques to measure fair value that maximize the use of observable inputs and minimize the use of unobservable inputs. These inputs are prioritized as follows:

Level 1 — Quoted unadjusted prices for identical instruments in active markets.

Level 2 — Quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-derived valuations in which all observable inputs and significant value drivers are observable in active markets.

Level 3 — Model-derived valuations in which one or more significant inputs or significant value drivers are unobservable, including assumptions developed by the Company.

The valuation techniques that may be used to measure fair value are as follows:

Market Approach — Uses prices and other relevant information generated by market transactions involving identical or comparable assets or liabilities.

Income Approach — Uses valuation techniques to convert future amounts to a single present amount based on current market expectations about those future amounts, including present value techniques, option pricing models, and excess earnings method.

Cost Approach — Based on the amount that currently would be required to replace the service capacity of an asset (replacement cost).

The following tables present, for each of the fair value levels, the Company’s assets that are measured at fair value on a recurring basis at September 30, 2017 and at December 31, 2016:

	At September 30, 2017
	Quoted Prices in Active Markets for Identical Assets (Level 1 Inputs)		Significant Other Observable Inputs (Level 2 Inputs)		Significant Unobservable Inputs (Level 3 Inputs)		Total
Assets:
Cash equivalents:
Money market funds	$	10,627	$	—	$	—	$	10,627
Investments:
Certificates of deposit		—		60,000		—		60,000
Total	$	10,627	$	60,000	$	—	$	70,627

	At December 31, 2016
	Quoted Prices in Active Markets for Identical Assets (Level 1 Inputs)		Significant Other Observable Inputs (Level 2 Inputs)		Significant Unobservable Inputs (Level 3 Inputs)		Total
Assets:
Investments:
Certificates of deposit	$	—	$	44,774	$	—	$	44,774
Total	$	—	$	44,774	$	—	$	44,774

The Company measures eligible assets and liabilities at fair value with changes in value recognized in earnings. Fair value treatment may be elected either upon initial recognition of an eligible asset or liability or, for an existing asset or liability, if an event triggers a new basis of accounting. The Company did not elect to remeasure any of its existing financial assets or liabilities, and did not elect the fair value option for any financial assets and liabilities transacted in the nine months ended September 30, 2017 or the year ended December 31, 2016.

The Company considers all highly liquid investments with an original maturity of three months or less at the date of purchase to be cash equivalents. Investments not classified as cash equivalents with maturities less than one year from the balance sheet date are classified as short-term investments, while investments with maturities in excess of one year from the balance sheet date are classified as long-term investments. Management determines the appropriate classification of investments at the time of purchase, and re-evaluates such determination at each balance sheet date.

Cash and cash equivalents primarily consist of cash on deposit with banks, and amounts held in interest-bearing money market accounts. Cash equivalents are carried at cost, which approximates their fair market value.

The following is a summary of cash, cash equivalents, and investments as of September 30, 2017 and December 30, 2016.


	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
September 30, 2017
Cash and cash equivalents due in 90 days or less	$	25,636	$	—	$	—	$	25,636
Investments:
Certificates of deposit due in one year or less		60,000					$	60,000
Total cash, cash equivalents, and investments	$	85,636	$	—	$	—	$	85,636


	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
December 31, 2016
Cash and cash equivalents due in 90 days or less	$	29,476	$	—	$	—	$	29,476
Investments:
Certificates of deposit due in one year or less		44,774					$	44,774
Total cash, cash equivalents, and investments	$	74,250	$	—	$	—	$	74,250

Certificates of deposit at September 30, 2017 had maturity dates ranging from nine to twelve months. Certificates of deposit at December 31, 2016 had maturity dates ranging from six to twelve months.

Property and Equipment, Net

9 Months Ended

Sep. 30, 2017

Property Plant And Equipment [Abstract]

Property and Equipment, Net

4. Property and Equipment, Net

Property and equipment consists of the following:

	At September 30, 2017			At December 31, 2016
Computer equipment		3,009			2,001
Capitalized software		174			114
Website development costs		4,167			2,680
Furniture and fixtures		4,461			3,386
Leasehold improvements		10,646			8,202
Construction in progress		35			119
		22,492			16,502
Less accumulated depreciation		(6,392	)		(3,722	)
Property and equipment, net	$	16,100		$	12,780

Depreciation and amortization expense on property and equipment for the three months ended September 30, 2017 and 2016 and for the nine months ended September 30, 2017 and 2016 was $1,083, $545, $2,670, and $1,381 respectively.

Other Long-Term Assets

9 Months Ended

Sep. 30, 2017

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

Other Long-Term Assets

5. Other Long-Term Assets

Other long-term assets consists of the following:

	At September 30, 2017		At December 31, 2016
Deferred IPO issuance costs	$	4,142	$	—
Other long-term assets		16		—
	$	4,158	$	—

Deferred IPO issuance costs, which primarily consist of direct incremental legal and accounting fees relating to the IPO, are capitalized. The deferred issuance costs will be offset against IPO proceeds upon the completion of the Company's IPO, which occurred in October 2017. As of September 30, 2017, $4,142 of deferred IPO issuance costs were recorded in other long-term assets in the accompanying unaudited condensed consolidated balance sheet. As of December 31, 2016, there were no deferred IPO issuance costs recorded.

Accrued Expenses

9 Months Ended

Sep. 30, 2017

Accrued Liabilities Current [Abstract]

Accrued Expenses

6. Accrued Expenses

Accrued expenses consists of the following:

	At September 30, 2017		At December 31, 2016
Accrued bonuses	$	5,086	$	4,662
Accrued commissions		1,520		1,305
Other accrued expenses		3,347		2,417
	$	9,953	$	8,384

Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Operating Leases The Company’s lease obligations consist of various leases for office space in Massachusetts and Dublin with various lease terms through January 2024. The terms of the Company’s Massachusetts lease agreements provide for rental payments that increase on an annual basis. The Company recognizes rent expense on a straight-line basis over the lease period. The Company does not have any debt or material capital lease obligations as of December 31, 2016 and all of the Company’s property, equipment, and software have been purchased with cash. The Company has no material long-term purchase obligations outstanding with any vendors or third parties. As of September 30, 2017, there were no material changes in the Company’s contractual obligations and commitments from those disclosed in the Prospectus filed with the SEC, other than as discussed below. On September 26, 2017, the Company entered into a lease for approximately 13,326 square feet of rentable office space located at Upper Hatch Street, Dublin, Ireland. The lease ends on August 11, 2023. Excluding operating costs, rent payments for the first year is $0.5 million. In August 2018, the rent amount is subject to a national rent review, which will establish the rent payments for the remainder of the lease. At September 30, 2017 and December 31, 2016, restricted cash was $1,783 and $2,044, respectively. Restricted cash for both periods was held at a financial institution in an interest-bearing cash account as collateral for two letters of credit related to the contractual provisions of the Company's building leases that require security deposits. Legal Matters The Company, from time to time, is party to litigation arising in the ordinary course of business. Management does not believe that the outcome of these claims will have a material adverse effect on the consolidated financial position, results of operations or cash flows of the Company based on the status of proceedings at this time. Guarantees and Indemnification Obligations In the ordinary course of business, the Company enters into agreements with its customers that are consistent with industry practice with respect to licensing, infringement, indemnification, and other standard provisions. The Company does not, in the ordinary course, agree to indemnification obligations for the Company under its contracts with customers. Based on historical experience and information known at September 30, 2017 and December 31, 2016, the Company has not incurred any costs for guarantees or indemnities.

Convertible Preferred Stock and Stockholders' Equity

9 Months Ended

Sep. 30, 2017

Equity [Abstract]

Convertible Preferred Stock and Stockholders' Equity

8. Convertible Preferred Stock and Stockholders’ Equity

On June 21, 2017, the Company amended and restated its Certificate of Incorporation pursuant to the Third Amended and Restated Certificate of Incorporation. Under the Third Amended and Restated Certificate of Incorporation, the total number of shares of all classes of stock which the Company shall have authority to issue is (i) 120,020,700 shares of Class A common stock, par value $0.001 per share, (ii) 80,013,800 shares of Class B common stock, par value $0.001 per share, and (iii) 11,091,782 shares of Preferred Stock, par value $0.001 per share, of which 3,333,000 shares are designated Series A Preferred Stock, 3,329,497 shares are designated Series B Preferred Stock, 1,648,978 shares are designated Series C Preferred Stock, 1,673,105 shares are designated Series D convertible preferred stock, or Series D Preferred Stock, and 1,107,202 shares are designated Series E convertible preferred stock, or Series E Preferred Stock. The Series A Preferred Stock, Series B Preferred Stock, Series C Preferred Stock, Series D Preferred Stock, and Series E Preferred Stock are referred to collectively as the Preferred Stock.

Upon the effectiveness of the Third Amended and Restated Certificate of Incorporation, (i) each share of Class A common stock issued and outstanding was recapitalized, reclassified, and reconstituted into two (2) fully paid and non-assessable shares of outstanding Class A common stock and four (4) fully paid and non-assessable shares of outstanding Class B common stock, and (ii) each share of Class B common stock of the Corporation issued and outstanding was recapitalized, reclassified, and reconstituted into two (2) fully paid and non-assessable shares of outstanding Class A common stock and four (4) fully paid and non-assessable shares of outstanding Class B common stock.

Further, upon the effectiveness of the Third Amended and Restated Certificate of Incorporation, the number of shares of common stock as to which each outstanding option to purchase common stock is exercisable for and each outstanding restricted stock unit (“RSU”) is convertible into was adjusted such that upon exercise of outstanding stock options or vesting of outstanding RSUs, each holder will receive two (2) fully paid and non-assessable shares of Class A common stock and four (4) fully paid and non-assessable shares of Class B common stock in respect of each share of common stock previously underlying such option or RSU. The exercise price per share of common stock underlying each outstanding option was adjusted upon the effectiveness of the Third Amended and Restated Certificate of Incorporation to be one-sixth of the exercise price per share in effect immediately prior to such adjustment and the fair market value per share of common stock issuable upon settlement of such RSU was adjusted to be one-sixth of the fair market value per share in effect immediately prior to the recapitalization.

All share and per share data shown in the accompanying Unaudited Condensed Consolidated Financial Statements and related notes have been retroactively revised to reflect the share recapitalization.

On October 16, 2017, in connection with the closing of the Company’s IPO, each share of Preferred Stock was converted into approximately six shares of Class A common stock pursuant to the terms of the Third Amended and Restated Certificate of Incorporation. Immediately following the closing of the Company’s IPO and the conversion of the Preferred Stock, the Company’s Fourth Amended and Restated Certificate of Incorporation became effective. Under the Fourth Amended and Restated Certificate of Incorporation, the capital structure of the Company was adjusted. See Note 12 to these Unaudited Condensed Consolidated Financial Statements for additional information regarding the conversion of the Preferred Stock and the impact of the effectiveness of the Company’s Fourth Amended and Restated Certificate of Incorporation.

Common Stock

Each share of Class A common stock entitles the holder to one (1) vote for each share on all matters submitted to a vote of the Company’s stockholders at all meetings of stockholders and written actions in lieu of meetings. Each share of Class B common stock entitles the holder to ten votes for each share on all matters submitted to a vote of the Company’s stockholders at all meetings of stockholders and written actions in lieu of meetings.

Holders of common stock are entitled to receive dividends, when and if declared by the Board.

At September 30, 2017, each share of Class B common stock was convertible into one share of Class A common stock at the option of the holder at any time. Automatic conversion was to occur upon the occurrence of a Transfer, as was defined in the Third Amended and Restated Certificate of Incorporation, of such share of Class B common stock. Upon either the death or voluntary termination of the Company’s Chief Executive Officer, all shares of Class B common stock will automatically be converted into one share of Class A common stock.

Upon the effectiveness of the Company’s Fourth Amended and Restated Certificate of Incorporation, additional terms of conversion and transfer were implemented. See Note 12 to these Unaudited Condensed Consolidated Financial Statements for additional information regarding the current conversion and transfer terms of the Company’s common stock.

Preferred Stock

On August 23, 2016, the Company completed a Series E Preferred Stock financing in the amount of $59,732, net of issuance costs of approximately $268. In connection with this issuance, the Company used the proceeds received to repurchase and retire certain outstanding shares of Series A, Series B, and Series C Preferred Stock and common stock, as well as certain vested stock options and RSUs, from existing stockholders in the fourth quarter of 2016. The difference between the amount implicitly paid to repurchase the various classes of Preferred Stock and the corresponding carrying value of the underlying shares ($32,087) was treated as a deemed dividend and was recorded against retained earnings. As the shares of common stock were repurchased for constructive retirement in the fourth quarter of 2016, the excess purchase price over the corresponding par value was charged directly to retained earnings.

The Company’s Preferred Stock at both September 30, 2017 and December 31, 2016 was as follows:

	Original Issue Price Per Share		Shares Authorized		Issued and Outstanding		Liquidation Amount		Carrying Value
Series A Preferred Stock	$	0.525053		3,333,000		2,824,703	$	1,483	$	1,483
Series B Preferred Stock	$	0.780899		3,329,497		2,938,486		2,295		2,295
Series C Preferred Stock	$	0.849012		1,648,978		1,550,612		1,316		1,316
Series D Preferred Stock	$	40.642989		1,673,105		1,673,105		68,000		67,872
Series E Preferred Stock	$	54.190650		1,107,202		1,107,202		60,000		59,732
				11,091,782		10,094,108	$	133,094	$	132,698

At both September 30, 2017 and December 31, 2016, 20,188,226 shares of Class A common stock and 40,376,452 shares of Class B common stock were reserved for conversion of the outstanding Preferred Stock.

Stock-based Compensation

9 Months Ended

Sep. 30, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

9. Stock-based Compensation

For the three months ended September 30, 2017 and 2016 total stock-based compensation expense was $74 and $88, respectively. For the nine months ended September 30, 2017 and 2016 total stock-based compensation expense was $224 and $236, respectively. All stock-based compensation expense for these periods related to stock options.

On January 1, 2017, the Company adopted ASU 2016-09 and elected to account for forfeitures when they occur, on a modified retrospective basis. The cumulative effect adjustment related to the Company's accounting policy change for forfeitures was not material.

Total stock-based compensation expense was allocated as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Cost of revenue	$	6	$	6	$	16	$	14
Sales and marketing expense		35		43		108		119
Product, technology, and development expense		24		28		72		76
General and administrative expense		9		11		28		27
Total	$	74	$	88	$	224	$	236

As of September 30, 2017, there was approximately $0.5 million of unrecognized stock-based compensation expense related to stock options which is expected to be recognized over 2.1 years.

In addition to stock options, the Company has historically granted RSUs which are subject to both a service-based vesting and a performance-based vesting condition achieved upon a liquidity event, defined as either a change of control or an IPO. As of September 30, 2017, total unrecognized stock-based compensation expense related to these RSUs was approximately $10.1 million and is expected to be recognized over 3.1 years. As of September 30, 2017, the Company had not recognized compensation cost related to stock-based awards with these performance conditions as the liquidity event had not occurred. The Securities and Exchange Commission’s declaration of effectiveness of the Company’s registration statement on Form S-1 on October 11, 2017 satisfied the liquidity event performance condition. The cumulative unrecognized stock-based compensation expense related to these awards was $2.5 million through October 11, 2017.

Earnings Per Share

9 Months Ended

Sep. 30, 2017

Earnings Per Share [Abstract]

Earnings Per Share

10. Earnings Per Share

Net income per share information is determined using the two-class method, which includes the weighted-average number of shares of common stock outstanding during the period and other securities that participate in dividends (a participating security). The Company considers the convertible Preferred Stock to be participating securities because they include rights to participate in dividends with the common stock.

Under the two-class method, basic net income per share attributable to common stockholders is computed by dividing the net income attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period. Diluted net income (loss) per share attributable to common stockholders is computed using the more dilutive of (1) the two-class method or (2) the if-converted method. The Company allocates net income first to preferred stockholders based on dividend rights under the Company’s certificate of incorporation and then to preferred and common stockholders based on ownership interests. Net losses are not allocated to preferred stockholders as they do not have an obligation to share in the Company’s net losses.

For all periods presented and as of the date of this Current Report on Form 10-Q, the Company had and has two classes of common stock outstanding: Class A common stock and Class B common stock. As more fully described in Note 8, the rights of the holders of Class A and Class B common stock were and are identical, except with respect to voting and conversion. Each share of Class A common stock was and is entitled to one (1) vote per share and each share of Class B common stock was and is entitled to ten (10) votes per share. Each share of Class B common stock was and is convertible into one share of Class A common stock at the option of the holder at any time. In addition, each share of Class B common stock was and is automatically convertible into one share of Class A common stock upon any transfer of such share, which is defined to include entering into a voting agreement, whether or not for value, except for certain transfers described in both the Company’s Third Amended and Restated Certificate of Incorporation and Fourth Amended and Restated Certificate of Incorporation, including, without limitation, transfers to certain family members of the transferor stockholder. Upon either the death or voluntary termination of the Company’s Chief Executive Officer, all shares of Class B common stock were and are automatically convertible into one share of Class A common stock. Upon the effectiveness of the Company’s Fourth Amended and Restated Certificate of Incorporation, additional terms of conversion and transfer were implemented. See Note 12 to these Unaudited Condensed Consolidated Financial Statements for additional information regarding the current conversion and transfer terms of the Company’s common stock.

Diluted net income (loss) per share gives effect to all potentially dilutive securities. Potential dilutive securities consist of shares of common stock issuable upon the exercise of stock options, shares of common stock issuable upon the vesting of RSUs, and shares of common stock issuable upon the conversion of the outstanding convertible preferred stock. The dilutive effect of these common stock equivalents is reflected in diluted earnings per share by application of the treasury stock method; however, outstanding RSUs, which are contingently issuable upon the achievement of a liquidity event have been excluded from the dilutive share calculation as it was not probable the vesting criteria for these awards would be met in any of the periods presented.

For the three and nine months ended September 30, 2017 and 2016, the two-class method was used in the computation of diluted net income per share, as the result was more dilutive.

The following table presents a reconciliation of the numerator and denominator used in the calculation of basic and diluted net income (loss) per share:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2017			2016			2017			2016
Numerator:
Net income	$	2,379		$	2,138		$	10,932		$	2,659
Net income attributable to participating securities		(1,401	)		(1,260	)		(6,446	)		(1,554	)
Net income attributable to common stockholders — basic	$	978		$	878		$	4,486		$	1,105
Net income	$	2,379		$	2,138		$	10,932		$	2,659
Net income attributable to participating securities		(1,345	)		(1,222	)		(6,198	)		(1,507	)
Net income attributable to common stockholders — diluted	$	1,034		$	916		$	4,734		$	1,152
Denominator:
Weighted-average number of shares of common stock used in computing net income per share attributable to common stockholders — basic		42,262,035			44,692,419			42,168,904			44,665,063
Dilutive effect of share equivalents resulting from stock options		4,305,138			3,376,954			4,141,726			3,375,691
Weighted-average number of shares of common stock used in computing net income per share — diluted		46,567,173			48,069,373			46,310,630			48,040,754
Net income per share attributable to common stockholders:
Basic	$	0.02		$	0.02		$	0.11		$	0.02
Diluted	$	0.02		$	0.02		$	0.10		$	0.02

The following potentially dilutive common stock equivalents have been excluded from the calculation of diluted weighted-average shares outstanding for the three and nine months ended September 30, 2017 and 2016, as their effect would have been anti-dilutive for the periods presented:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Stock options outstanding		—		1,332,666		816,318		1,226,182
Restricted stock units outstanding		2,353,152		1,124,694		2,353,152		1,124,694
Convertible preferred stock		—		—		—		—

Segment and Geographic Information

9 Months Ended

Sep. 30, 2017

Segment Reporting [Abstract]

Segment and Geographic Information

11. Segment and Geographic Information

The Company has two reportable segments, United States and International. Segment information is presented in the same manner as the Company’s chief operating decision maker (“CODM”), reviews the Company’s operating results in assessing performance and allocating resources. The CODM reviews revenue and operating income (loss) for each reportable segment as a proxy for the operating performance of the Company’s United States and International operations. The Company’s Chief Executive Officer is the CODM on behalf of both reportable segments.

The United States segment derives revenues from marketplace subscriptions, advertising services, and other revenues from customers within the United States. The International segment derives revenues from marketplace subscriptions, advertising services, and other revenues from customers outside of the United States. A majority of the Company’s operational overhead expenses, including technology and personnel costs, and other general and administrative costs associated with running the Company’s business, are incurred in the United States and not allocated to the international segment. Assets and costs discretely incurred by reportable segments, including depreciation and amortization, are included in the calculation of reportable segment (loss) income from operations. Segment operating income (loss) does not reflect the transfer pricing adjustments related to the Company’s foreign subsidiaries, which are recorded for statutory reporting purposes. Asset information is assessed and reviewed on a global basis.

Information regarding the Company’s operations by segment and geographical area is presented as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Segment revenue:
United States	$	80,394	$	52,435	$	219,954	$	136,195
International		2,595		701		6,310		1,182
Total revenue	$	82,989	$	53,136	$	226,264	$	137,377

	Three Months Ended September 30,						Nine Months Ended September 30,
	2017			2016			2017			2016
Segment (loss) income from operations:
United States	$	9,337		$	8,351		$	33,617		$	16,818
International		(6,474	)		(5,094	)		(18,375	)		(12,853	)
Total income from operations	$	2,863		$	3,257		$	15,242		$	3,965

As of September 30, 2017 and December 31, 2016, property and equipment held outside the United States was not material.

Subsequent Events	9 Months Ended
Subsequent Events	Sep. 30, 2017
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events On October 16, 2017, the Company completed the IPO, in which the Company issued and sold 3,205,000 shares of its Class A common stock, including the full exercise by the underwriters of their option to purchase 705,000 shares of Class A common stock, at a public offering price of $16.00 per share for aggregate gross proceeds of $51.3 million. The Company received approximately $43.0 million in net proceeds after deducting $3.6 million of underwriting discounts and commissions and approximately $4.7 million in offering costs. In addition to shares of Class A common stock issued and sold by the Company, certain selling shareholders sold an aggregate of 7,605,000 shares of Class A common stock, including the full exercise by the underwriters of their option to purchase 705,000 shares of Class A common stock, as part of the IPO. Upon the closing of the IPO, all of the outstanding shares of convertible Preferred Stock automatically converted into 20,188,226 shares of Class A common stock and 40,376,452 shares of Class B common stock. The 40,376,452 shares of Class B common stock subsequently converted into 40,376,452 shares of Class A common stock resulting in a total conversion of all outstanding shares of Preferred Stock into 60,564,678 shares of Class A common stock. Subsequent to the closing of the IPO, there were no shares of Preferred Stock outstanding. Allen & Company LLC acted as an underwriter in the IPO. Immediately prior to the IPO, Allen & Company LLC and its associated persons, including Ian Smith, a member of the Company’s board of directors, beneficially owned shares of the Company’s outstanding preferred stock representing 13.5% of the Company’s outstanding preferred stock. In connection with Allen & Company LLC’s role as an underwriter in the IPO, pursuant to the underwriting agreement, Allen & Company LLC purchased 2,190,200 shares of our Class A common stock in the IPO at $14.88 per share for a total purchase price of $32,590,176, after deducting underwriting discounts and commissions paid to Allen & Company LLC of $2,453,024. Receipt of such proceeds may be deemed to be a related person transaction pursuant to the Company’s related person transaction policy. In connection with the IPO, in October 2017, the Company’s board of directors adopted, and the Company’s stockholders approved, the Omnibus Equity Compensation Plan (the “2017 Plan”) for the purpose of granting incentive stock options, non-qualified stock options, restricted stock, restricted stock units, and other share-based awards to employees, non-employee directors, and consultants. The 2017 Plan is the successor to the Amended and Restated 2015 Equity Incentive Plan (the “2015 Plan”). In conjunction with the adoption of the 2017 Plan, options and RSUs from the 2015 Plan will remain outstanding but no additional grants will be made from the 2015 Plan. Also, in connection with the closing of the IPO, each share of Series A Preferred Stock automatically converted into 6.0000023 shares of common stock, each share of Series B Preferred Stock converted into 6.0000015 shares of common stock, each share of Series C Preferred Stock converted into 6 shares of common stock, each share of Series D Preferred Stock converted into 6.0000159 shares of common stock and each share of Series E Preferred Stock converted into 6 shares of common stock, one-third of which number of shares of common stock was Class A common stock and two thirds of which number of shares of common stock was Class B common stock. Each such share of Class B common stock issued upon conversion of the Preferred Stock was then immediately converted into one share of Class A common stock. Immediately following such conversion, the Company’s Fourth Amended and Restated Certificate of Incorporation became effective. Pursuant to the Fourth Amended and Restated Certificate of Incorporation, the Company is authorized to issue 500,000,000 shares of Class A common stock, 100,000,000 shares of Class B common stock, and 10,000,000 shares of preferred stock, all with a par value of $0.001 per share. The preferred stock is currently undesignated and no preferred stock is outstanding. In addition, pursuant to the Fourth Amended and Restated Certificate of Incorporation, all shares of Class B common stock will automatically convert into shares of Class A common stock, on a share for share basis, upon the date falling after the first to occur of the death of Langley Steinert, the Company’s Chief Executive Officer, President and Chairman, his voluntary termination of all employment with the Company and service on the Company’s board of directors or the sum of the number of shares of capital stock held by Langley Steinert, by any Family Member of Langley Steinert, and by any Permitted Entity of Langley Steinert (as such terms are defined in the Fourth Amended and Restated Certificate of Incorporation), assuming the exercise and settlement in full of all outstanding options and convertible securities and calculated on an as-converted to Class A common stock basis, being less than 9,091,484. Shares of Class B common stock will not automatically convert into shares of Class A common stock upon the termination of Mr. Steinert's status as an officer and director, unless such termination is either made voluntarily by Mr. Steinert or due to Mr. Steinert's death. Once converted into Class A common stock, the converted shares of Class B common stock will not be reissued. In addition, if all shares of Class B common stock are converted into Class A common stock, then any outstanding options or convertible securities with the right to purchase or acquire shares of Class B common stock shall become a right to purchase or acquire shares of Class A common stock. On October 25, 2017, the Company granted a total of 762,704 RSUs with a grant date fair value of $29.01 per share. The total aggregate value of these RSUs is approximately $22.1 million.

Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying interim condensed consolidated financial statements (the “Unaudited Condensed Consolidated Financial Statements”) are unaudited. These Unaudited Condensed Consolidated Financial Statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2016 included in the Company’s final prospectus related to the initial public offering (“IPO”), filed with the Securities and Exchange Commission (“SEC”) pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended, on October 12, 2017 (the “Prospectus”). The Unaudited Condensed Consolidated Financial Statements have been prepared pursuant to the rules and regulations of the SEC. Accordingly, certain information and footnote disclosures normally included in financial statements prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. The Unaudited Condensed Consolidated Financial Statements reflect all adjustments, consisting of only normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of September 30, 2017 and December 31, 2016, results of operations for the three months and nine months ended September 30, 2017 and 2016, and cash flows for the nine months ended September 30, 2017 and 2016. These interim periods are not necessarily indicative of the results to be expected for any other interim period or the full year. The accompanying Unaudited Condensed Consolidated Financial Statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the Unaudited Condensed Consolidated Financial Statements. As of September 30, 2017, there have been no material changes in the Company's significant accounting policies from those that were disclosed in the Prospectus except as discussed below. In March 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-09, Compensation — Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting (“ASU 2016-09”). The guidance identifies areas for simplification involving several aspects of accounting for share-based payments, including income tax consequences, classification of awards as either equity or liabilities, an option to make a policy election to recognize gross stock-based compensation expense with actual forfeitures recognized as they occur, as well as certain classification changes on the statement of cash flows. The Company adopted ASU 2016-09 on January 1, 2017 and elected to account for forfeitures when they occur, on a modified retrospective basis. The cumulative effect adjustment related to the Company's accounting policy change for forfeitures was not material. In accordance with the adoption of this guidance, the tax effect of differences between tax deductions related to stock compensation and the corresponding financial statement expense compensation will no longer be recorded to additional paid-in capital in the balance sheet. Instead, such amounts will be recorded to tax expense. During the three and nine months ended September 30, 2017, the Company recorded tax benefits of $267 and $640, respectively, related to differences between tax deductions related to stock compensation and the corresponding financial statement expense compensation. The Company also elected to prospectively apply the change in presentation of excess tax benefits, wherein excess tax benefits recognized on stock-based compensation expense is now classified as an operating activity in the consolidated statements of cash flows. The Company did not adjust the classifications of excess tax benefits in its condensed consolidated statements of cash flows for the nine months ended September 30, 2016. The adoption did not have any other material impact on the Company's condensed consolidated financial statements.
Principles of Consolidation	Principles of Consolidation The accompanying Unaudited Condensed Consolidated Financial Statements include the accounts of CarGurus, Inc. and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. The Company prepares its Unaudited Condensed Consolidated Financial Statements and related disclosures in conformity with GAAP.
Subsequent Event Considerations	Subsequent Event Considerations The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure, other than those disclosed in this Quarterly Report on Form 10-Q.
Use of Estimates	Use of Estimates In preparing its Unaudited Condensed Consolidated Financial Statements in accordance with GAAP, the Company is required to make estimates and assumptions that affect the amounts of assets, liabilities, revenue, costs, and expenses, and disclosure of contingent assets and liabilities which are reported in the Unaudited Condensed Consolidated Financial Statements and accompanying disclosures. The accounting estimates that require the most difficult and subjective judgments include revenue recognition and revenue reserves, contingent liabilities, allowances for doubtful accounts, expected future cash flows used to evaluate the recoverability of long-lived assets, the expensing and capitalization of product, technology, and development costs for website development and internal-use software, the determination of the fair value of stock awards issued, stock-based compensation expense, and the recoverability of the Company's net deferred tax assets and related valuation allowance. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could materially differ from the Company’s estimates and assumptions.
Emerging Growth Company Status	Emerging Growth Company Status The Company is an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act, or JOBS Act, and may take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. The Company may take advantage of these exemptions until the Company is no longer an emerging growth company. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards. The Company has elected to use the extended transition period for complying with new or revised accounting standards and as a result of this election, its financial statements may not be comparable to companies that comply with public company effective dates. The Company may take advantage of these exemptions up until December 31, 2022 or such earlier time that it is no longer an emerging growth company. The Company would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue, it has more than $700.0 million in market value of its stock held by non-affiliates (and it has been a public company for at least 12 months, and has filed one annual report on Form 10-K), or it issues more than $1.0 billion of non-convertible debt securities over a three-year period.
Concentration of Credit Risk	Concentration of Credit Risk The Company has no significant off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, investments, and trade accounts receivable. The Company maintains its cash, cash equivalents, and investments principally with accredited financial institutions of high credit standing. Although the Company deposits its cash and investments with multiple financial institutions, its deposits, at times, may exceed federally insured limits. Credit risk with respect to accounts receivable is dispersed due to the large number of customers. The Company routinely assesses the creditworthiness of its customers. The Company generally has not experienced any material losses related to receivables from individual customers, or groups of customers. The Company does not require collateral. Due to these factors, no additional credit risk beyond amounts provided for collection losses is believed by management to be probable in the Company's accounts receivable. For the nine months ended September 30, 2017 and the year ended December 31, 2016, no individual customer accounted for more than 10% of total revenue. As of September 30, 2017, three customers accounted for 17%, 15%, and 10% of net accounts receivable, respectively. As of December 31, 2016, two customers accounted for 24% and 15% of net accounts receivable, respectively. No other individual customer accounted for more than 10% of net accounts receivable at September 30, 2017 or December 31, 2016.
Recent Accounting Pronouncements	Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by the Company on or prior to the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which modifies how all entities recognize revenue, and consolidates into one Accounting Standards Codification (“ASC”) Topic (ASC Topic 606, Revenue from Contracts with Customers) the current guidance found in ASC Topic 605, and various other revenue accounting standards for specialized transactions and industries. ASU 2014-09 outlines a comprehensive five-step revenue recognition model based on the principle that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 may be applied using either a full retrospective approach, under which all years included in the financial statements will be presented under the revised guidance, or a modified retrospective approach, under which financial statements will be prepared under the revised guidance for the year of adoption, but not for prior years. Under the latter method, entities will recognize a cumulative catch-up adjustment to the opening balance of retained earnings at the effective date for contracts that still require performance by the entity at the date of adoption. In August 2015, the FASB issued ASU No. 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of Effective Date (“ASU 2015-14”), which defers the effective date of ASU 2014-09 by one year. ASU 2014-09 is now effective for public entities for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. The Company has developed an implementation plan to adopt this new guidance. As part of this plan, the Company is currently assessing the impact of the new guidance on its results of operations. Based on the Company’s procedures performed to date, nothing has come to its attention that would indicate that the adoption of ASU 2014-09 will have a material impact on its revenue recognition; however, further analysis is required and the Company will continue to evaluate this assessment throughout 2017 and 2018. While the Company is still evaluating the impact that this guidance will have on its financial statements and related disclosures, the Company’s preliminary assessment is that there will be an impact relating to the accounting for costs to acquire a contract. Under ASU 2015-14, the Company will be required to capitalize certain costs, primarily commission expense to sales representatives, on its consolidated balance sheet and amortize such costs over the period of performance for the underlying customer contracts. The Company is still evaluating the impact of capitalizing costs to execute a contract. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”). ASU 2016-02 requires a lessee to recognize most leases on the balance sheet but recognize expenses on the income statement in a manner similar to current practice. The update states that a lessee will recognize a lease liability for the obligation to make lease payments and a right-to-use asset for the right to use the underlying assets for the lease term. Leases will continue to be classified as either financing or operating, with classification affecting the recognition, measurement, and presentation of expenses and cash flows arising from a lease. For public entities, the new standard is effective for interim and annual periods beginning on or after January 1, 2019, with early adoption permitted. For non-public entities, the new standard is effective for annual periods beginning after December 15, 2019, with early adoption permitted. The Company is evaluating the impact this guidance may have on its consolidated financial statements. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”). ASU 2016-15 is intended to add or clarify guidance on the classification of certain cash receipts and payments in the statement of cash flows and to eliminate the diversity in practice related to such classifications. For public entities, the guidance in ASU 2016-15 is required for annual reporting periods beginning after December 15, 2017, with early adoption permitted. For non-public entities, the guidance is effective for annual reporting periods beginning after December 15, 2018, with early adoption permitted. The Company is currently in the process of evaluating the impact and timing of adoption of ASU 2016-15 on its consolidated financial statements.
Fair Value of Financial Instruments Including Cash, Cash Equivalents and Investments	The Company considers all highly liquid investments with an original maturity of three months or less at the date of purchase to be cash equivalents. Investments not classified as cash equivalents with maturities less than one year from the balance sheet date are classified as short-term investments, while investments with maturities in excess of one year from the balance sheet date are classified as long-term investments. Management determines the appropriate classification of investments at the time of purchase, and re-evaluates such determination at each balance sheet date.

Fair Value of Financial Instruments Including Cash, Cash Equivalents and Investments (Tables)

9 Months Ended

Sep. 30, 2017

Fair Value Disclosures [Abstract]

Schedule of Fair Value Levels, Assets Measured at Fair Value on Recurring Basis

The following tables present, for each of the fair value levels, the Company’s assets that are measured at fair value on a recurring basis at September 30, 2017 and at December 31, 2016:

	At September 30, 2017
	Quoted Prices in Active Markets for Identical Assets (Level 1 Inputs)		Significant Other Observable Inputs (Level 2 Inputs)		Significant Unobservable Inputs (Level 3 Inputs)		Total
Assets:
Cash equivalents:
Money market funds	$	10,627	$	—	$	—	$	10,627
Investments:
Certificates of deposit		—		60,000		—		60,000
Total	$	10,627	$	60,000	$	—	$	70,627

	At December 31, 2016
	Quoted Prices in Active Markets for Identical Assets (Level 1 Inputs)		Significant Other Observable Inputs (Level 2 Inputs)		Significant Unobservable Inputs (Level 3 Inputs)		Total
Assets:
Investments:
Certificates of deposit	$	—	$	44,774	$	—	$	44,774
Total	$	—	$	44,774	$	—	$	44,774

Schedule of Cash, Cash Equivalents, and Investments

The following is a summary of cash, cash equivalents, and investments as of September 30, 2017 and December 30, 2016.


	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
September 30, 2017
Cash and cash equivalents due in 90 days or less	$	25,636	$	—	$	—	$	25,636
Investments:
Certificates of deposit due in one year or less		60,000					$	60,000
Total cash, cash equivalents, and investments	$	85,636	$	—	$	—	$	85,636


	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
December 31, 2016
Cash and cash equivalents due in 90 days or less	$	29,476	$	—	$	—	$	29,476
Investments:
Certificates of deposit due in one year or less		44,774					$	44,774
Total cash, cash equivalents, and investments	$	74,250	$	—	$	—	$	74,250

Property and Equipment, Net (Tables)

9 Months Ended

Sep. 30, 2017

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment consists of the following:

	At September 30, 2017			At December 31, 2016
Computer equipment		3,009			2,001
Capitalized software		174			114
Website development costs		4,167			2,680
Furniture and fixtures		4,461			3,386
Leasehold improvements		10,646			8,202
Construction in progress		35			119
		22,492			16,502
Less accumulated depreciation		(6,392	)		(3,722	)
Property and equipment, net	$	16,100		$	12,780

Other Long-Term Assets (Tables)

9 Months Ended

Sep. 30, 2017

Other Assets Noncurrent Disclosure [Abstract]

Schedule of Other Long-Term Assets

Other long-term assets consists of the following:

	At September 30, 2017		At December 31, 2016
Deferred IPO issuance costs	$	4,142	$	—
Other long-term assets		16		—
	$	4,158	$	—

Accrued Expenses (Tables)

9 Months Ended

Sep. 30, 2017

Accrued Liabilities Current [Abstract]

Schedule of Accrued Expenses

Accrued expenses consists of the following:

	At September 30, 2017		At December 31, 2016
Accrued bonuses	$	5,086	$	4,662
Accrued commissions		1,520		1,305
Other accrued expenses		3,347		2,417
	$	9,953	$	8,384

Convertible Preferred Stock and Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2017

Equity [Abstract]

Summary of Preferred Stock

The Company’s Preferred Stock at both September 30, 2017 and December 31, 2016 was as follows:

	Original Issue Price Per Share		Shares Authorized		Issued and Outstanding		Liquidation Amount		Carrying Value
Series A Preferred Stock	$	0.525053		3,333,000		2,824,703	$	1,483	$	1,483
Series B Preferred Stock	$	0.780899		3,329,497		2,938,486		2,295		2,295
Series C Preferred Stock	$	0.849012		1,648,978		1,550,612		1,316		1,316
Series D Preferred Stock	$	40.642989		1,673,105		1,673,105		68,000		67,872
Series E Preferred Stock	$	54.190650		1,107,202		1,107,202		60,000		59,732
				11,091,782		10,094,108	$	133,094	$	132,698

Summary of Allocation of Stock-based Compensation Expense (Tables)

9 Months Ended

Sep. 30, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Allocation of Stock-based Compensation Expense

Total stock-based compensation expense was allocated as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Cost of revenue	$	6	$	6	$	16	$	14
Sales and marketing expense		35		43		108		119
Product, technology, and development expense		24		28		72		76
General and administrative expense		9		11		28		27
Total	$	74	$	88	$	224	$	236

Earnings Per Share (Tables)

9 Months Ended

Sep. 30, 2017

Earnings Per Share [Abstract]

Schedule of Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net (Loss) Income Per Share

The following table presents a reconciliation of the numerator and denominator used in the calculation of basic and diluted net income (loss) per share:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2017			2016			2017			2016
Numerator:
Net income	$	2,379		$	2,138		$	10,932		$	2,659
Net income attributable to participating securities		(1,401	)		(1,260	)		(6,446	)		(1,554	)
Net income attributable to common stockholders — basic	$	978		$	878		$	4,486		$	1,105
Net income	$	2,379		$	2,138		$	10,932		$	2,659
Net income attributable to participating securities		(1,345	)		(1,222	)		(6,198	)		(1,507	)
Net income attributable to common stockholders — diluted	$	1,034		$	916		$	4,734		$	1,152
Denominator:
Weighted-average number of shares of common stock used in computing net income per share attributable to common stockholders — basic		42,262,035			44,692,419			42,168,904			44,665,063
Dilutive effect of share equivalents resulting from stock options		4,305,138			3,376,954			4,141,726			3,375,691
Weighted-average number of shares of common stock used in computing net income per share — diluted		46,567,173			48,069,373			46,310,630			48,040,754
Net income per share attributable to common stockholders:
Basic	$	0.02		$	0.02		$	0.11		$	0.02
Diluted	$	0.02		$	0.02		$	0.10		$	0.02

Schedule of Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Weighted-average Shares Outstanding

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Stock options outstanding		—		1,332,666		816,318		1,226,182
Restricted stock units outstanding		2,353,152		1,124,694		2,353,152		1,124,694
Convertible preferred stock		—		—		—		—

Segment and Geographic Information (Tables)

9 Months Ended

Sep. 30, 2017

Segment Reporting [Abstract]

Summary of Operations by Segment and Geographical Area

Information regarding the Company’s operations by segment and geographical area is presented as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Segment revenue:
United States	$	80,394	$	52,435	$	219,954	$	136,195
International		2,595		701		6,310		1,182
Total revenue	$	82,989	$	53,136	$	226,264	$	137,377

	Three Months Ended September 30,						Nine Months Ended September 30,
	2017			2016			2017			2016
Segment (loss) income from operations:
United States	$	9,337		$	8,351		$	33,617		$	16,818
International		(6,474	)		(5,094	)		(18,375	)		(12,853	)
Total income from operations	$	2,863		$	3,257		$	15,242		$	3,965

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Tax benefits	$ (590,000)	$ (1,226,000)	$ (4,633,000)	$ (1,566,000)
Emerging growth company description			The Company would cease to be an emerging growth company if it has more than $1.07 billion in annual revenue, it has more than $700.0 million in market value of its stock held by non-affiliates (and it has been a public company for at least 12 months, and has filed one annual report on Form 10-K), or it issues more than $1.0 billion of non-convertible debt securities over a three-year period
Debt security maturity period			3 years
Description of significant off-balance sheet risk			The Company has no significant off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements.
Sales Revenue, Net \| Concentration of Credit Risk
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk, percentage			10.00%		10.00%
Net Accounts Receivable \| Concentration of Credit Risk \| Customer One
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk, percentage			17.00%		24.00%
Net Accounts Receivable \| Concentration of Credit Risk \| Customer Two
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk, percentage			15.00%		15.00%
Net Accounts Receivable \| Concentration of Credit Risk \| Customer Three
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk, percentage			10.00%
Minimum
Summary Of Significant Accounting Policies [Line Items]
Annual gross revenue			$ 1,070,000,000
Market value of common stock held by non-affiliates	700,000,000		700,000,000
Non convertible debt securities			1,000,000,000
Accounting Standards Update 2016-09
Summary Of Significant Accounting Policies [Line Items]
Tax benefits	$ 267,000		$ 640,000

Fair Value of Financial Instruments Including Cash, Cash Equivalents and Investments - Schedule of Fair Value Levels, Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value Measurements Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 70,627	$ 44,774
Quoted Prices in Active Markets for Identical Assets (Level 1 Inputs)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	10,627
Significant Other Observable Inputs (Level 2 Inputs)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	60,000	44,774
Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	10,627
Money Market Funds \| Quoted Prices in Active Markets for Identical Assets (Level 1 Inputs)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	10,627
Certificates of Deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments:	60,000	44,774
Certificates of Deposit \| Significant Other Observable Inputs (Level 2 Inputs)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments:	$ 60,000	$ 44,774

Fair Value of Financial Instruments Including Cash, Cash Equivalents and Investments - Schedule of Cash, Cash Equivalents, and Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Cash And Cash Equivalents [Line Items]
Cash and cash equivalents due in 90 days or less	$ 25,636	$ 29,476
Certificates of deposit due in one year or less	60,000	44,774
Total cash, cash equivalents, and investments	85,636	74,250
Estimated Fair Value
Cash And Cash Equivalents [Line Items]
Cash and cash equivalents due in 90 days or less	25,636	29,476
Certificates of deposit due in one year or less	60,000	44,774
Total cash, cash equivalents, and investments	$ 85,636	$ 74,250

Fair Value of Financial Instruments Including Cash, Cash Equivalents and Investments - Additional Information (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity of certificates of deposit, description	Certificates of deposit at September 30, 2017 had maturity dates ranging from nine to twelve months. Certificates of deposit at December 31, 2016 had maturity dates ranging from six to twelve months.
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity period of certificates of deposit	9 months	6 months
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity period of certificates of deposit	12 months	12 months

Property and Equipment, Net - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 22,492	$ 16,502
Less accumulated depreciation	(6,392)	(3,722)
Property and equipment, net	16,100	12,780
Computer Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	3,009	2,001
Capitalized Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	174	114
Website Development Costs
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,167	2,680
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,461	3,386
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	10,646	8,202
Construction in Progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 35	$ 119

Other Long-Term Assets - Schedule of Other Long-Term Assets (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Deferred IPO issuance costs	$ 4,142,000	$ 0
Other long-term assets	16,000
Total other long-term assets	$ 4,158,000

Accrued Expenses - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accrued Liabilities Current [Abstract]
Accrued bonuses	$ 5,086	$ 4,662
Accrued commissions	1,520	1,305
Other accrued expenses	3,347	2,417
Accrued expenses, Total	$ 9,953	$ 8,384

Commitments and Contingencies - Additional Information (Details)		9 Months Ended	12 Months Ended
	Sep. 26, 2017 USD ($) ft²	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitments And Contingencies [Line Items]
Restricted cash		$ 1,783,000	$ 2,044,000
Cost for guarantees and indemnities		$ 0	$ 0
Upper Hatch Street, Dublin, Ireland
Commitments And Contingencies [Line Items]
Square feet of rentable office space \| ft²	13,326
Lease expiration date	Aug. 11, 2023
Rent expense	$ 500,000

Convertible Preferred Stock and Stockholders' Equity - Additional Information (Details) - $ / shares		9 Months Ended
	Jun. 21, 2017	Sep. 30, 2017	Oct. 16, 2017	Dec. 31, 2016
Class Of Stock [Line Items]
Preferred stock, shares authorized	11,091,782	11,091,782		11,091,782
Preferred stock, par value	$ 0.001
Common stock exercise feature		the number of shares of common stock as to which each outstanding option to purchase common stock is exercisable for and each outstanding restricted stock unit (“RSU”) is convertible into was adjusted such that upon exercise of outstanding stock options or vesting of outstanding RSUs, each holder will receive two (2) fully paid and non-assessable shares of Class A common stock and four (4) fully paid and non-assessable shares of Class B common stock in respect of each share of common stock previously underlying such option or RSU
Percentage of common stock exercise price adjusted	0.167%
Percentage of common stock fair value adjusted prior to recapitalization	0.167%
Class A Common Stock
Class Of Stock [Line Items]
Common stock, shares authorized	120,020,700	120,020,700		120,020,700
Common stock, par value	$ 0.001	$ 0.001		$ 0.001
Common stock conversion terms		each share of Class A common stock issued and outstanding was recapitalized, reclassified, and reconstituted into two (2) fully paid and non-assessable shares of outstanding Class A common stock and four (4) fully paid and non-assessable shares of outstanding Class B common stock
Number of shares issued upon exercise of outstanding stock options or vesting of outstanding RSUs	2
Convertible preferred stock, terms of conversion		in connection with the closing of the Company’s IPO, each share of Preferred Stock was converted into approximately six shares of Class A common stock pursuant to the terms of the Third Amended and Restated Certificate of Incorporation.
Class A Common Stock \| Subsequent Event
Class Of Stock [Line Items]
Common stock, shares authorized			500,000,000
Common stock, par value			$ 0.001
Convertible preferred stock converted into common stock			6
Class A Common Stock \| Class A Common Stock Recapitalized, Reclassified And Reconstituted [Member]
Class Of Stock [Line Items]
Common stock issued in conversion	2
Class A Common Stock \| Class B Common Stock Recapitalized, Reclassified And Reconstituted [Member]
Class Of Stock [Line Items]
Common stock issued in conversion	2
Class B Common Stock
Class Of Stock [Line Items]
Common stock, shares authorized	80,013,800	80,013,800		80,013,800
Common stock, par value	$ 0.001	$ 0.001		$ 0.001
Common stock conversion terms		each share of Class B common stock of the Corporation issued and outstanding was recapitalized, reclassified, and reconstituted into two (2) fully paid and non-assessable shares of outstanding Class A common stock and four (4) fully paid and non-assessable shares of outstanding Class B common stock.
Number of shares issued upon exercise of outstanding stock options or vesting of outstanding RSUs	4
Class B Common Stock \| Subsequent Event
Class Of Stock [Line Items]
Common stock, shares authorized			100,000,000
Common stock, par value			$ 0.001
Class B Common Stock \| Class A Common Stock Recapitalized, Reclassified And Reconstituted [Member]
Class Of Stock [Line Items]
Common stock issued in conversion	4
Class B Common Stock \| Class B Common Stock Recapitalized, Reclassified And Reconstituted [Member]
Class Of Stock [Line Items]
Common stock issued in conversion	4
Series A Preferred Stock
Class Of Stock [Line Items]
Preferred stock, shares authorized	3,333,000	3,333,000		3,333,000
Series B Preferred Stock
Class Of Stock [Line Items]
Preferred stock, shares authorized	3,329,497	3,329,497		3,329,497
Series C Preferred Stock
Class Of Stock [Line Items]
Preferred stock, shares authorized	1,648,978	1,648,978		1,648,978
Series D Preferred Stock
Class Of Stock [Line Items]
Preferred stock, shares authorized	1,673,105	1,673,105		1,673,105
Series E Preferred Stock
Class Of Stock [Line Items]
Preferred stock, shares authorized	1,107,202	1,107,202		1,107,202

Convertible Preferred Stock and Stockholders' Equity - Common Stock - Additional Information (Details)	9 Months Ended
	Sep. 30, 2017 Vote shares
Class Of Stock [Line Items]
Common stock conversion feature	each share of Class B common stock was convertible into one share of Class A common stock at the option of the holder at any time.
Common stock automatic conversion basis	Upon either the death or voluntary termination of the Company’s Chief Executive Officer, all shares of Class B common stock will automatically be converted into one share of Class A common stock.
Class A Common Stock
Class Of Stock [Line Items]
Common stock voting rights	one (1) vote for each share
Number of votes entitled to stockholders per share \| Vote	1
Class B Common Stock
Class Of Stock [Line Items]
Common stock voting rights	ten votes for each share
Number of votes entitled to stockholders per share \| Vote	10
Class of share converted to another class	one share of Class A common stock
Conversion of stock \| shares	1
Class B Common Stock \| Chief Executive Officer
Class Of Stock [Line Items]
Class of share converted to another class	one share of Class A common stock
Conversion of stock \| shares	1

Convertible Preferred Stock and Stockholders' Equity - Preferred Stock - Additional Information (Details) - USD ($) $ in Thousands	Aug. 23, 2016	Sep. 30, 2017	Dec. 31, 2016
Series E Preferred Stock
Class Of Stock [Line Items]
Financing amount, net of issuance costs	$ 59,732
Stock issuance costs	268
Series E Preferred Stock \| Retained Earnings
Class Of Stock [Line Items]
Deemed dividend	$ (32,087)
Class A Common Stock
Class Of Stock [Line Items]
Common stock reserved for conversion of outstanding preferred stock		20,188,226	20,188,226
Class B Common Stock
Class Of Stock [Line Items]
Common stock reserved for conversion of outstanding preferred stock		40,376,452	40,376,452

Convertible Preferred Stock and Stockholders' Equity - Summary of Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2017	Jun. 21, 2017	Dec. 31, 2016
Temporary Equity [Line Items]
Shares Authorized	11,091,782	11,091,782	11,091,782
Issued	10,094,108		10,094,108
Outstanding	10,094,108		10,094,108
Liquidation Amount	$ 133,094		$ 133,094
Convertible preferred stock (Note 8)	$ 132,698		$ 132,698
Series A Preferred Stock
Temporary Equity [Line Items]
Original Issue Price Per Share	$ 0.525053		$ 0.525053
Shares Authorized	3,333,000	3,333,000	3,333,000
Issued	2,824,703		2,824,703
Outstanding	2,824,703		2,824,703
Liquidation Amount	$ 1,483		$ 1,483
Convertible preferred stock (Note 8)	$ 1,483		$ 1,483
Series B Preferred Stock
Temporary Equity [Line Items]
Original Issue Price Per Share	$ 0.780899		$ 0.780899
Shares Authorized	3,329,497	3,329,497	3,329,497
Issued	2,938,486		2,938,486
Outstanding	2,938,486		2,938,486
Liquidation Amount	$ 2,295		$ 2,295
Convertible preferred stock (Note 8)	$ 2,295		$ 2,295
Series C Preferred Stock
Temporary Equity [Line Items]
Original Issue Price Per Share	$ 0.849012		$ 0.849012
Shares Authorized	1,648,978	1,648,978	1,648,978
Issued	1,550,612		1,550,612
Outstanding	1,550,612		1,550,612
Liquidation Amount	$ 1,316		$ 1,316
Convertible preferred stock (Note 8)	$ 1,316		$ 1,316
Series D Preferred Stock
Temporary Equity [Line Items]
Original Issue Price Per Share	$ 40.642989		$ 40.642989
Shares Authorized	1,673,105	1,673,105	1,673,105
Issued	1,673,105		1,673,105
Outstanding	1,673,105		1,673,105
Liquidation Amount	$ 68,000		$ 68,000
Convertible preferred stock (Note 8)	$ 67,872		$ 67,872
Series E Preferred Stock
Temporary Equity [Line Items]
Original Issue Price Per Share	$ 54.190650		$ 54.190650
Shares Authorized	1,107,202	1,107,202	1,107,202
Issued	1,107,202		1,107,202
Outstanding	1,107,202		1,107,202
Liquidation Amount	$ 60,000		$ 60,000
Convertible preferred stock (Note 8)	$ 59,732		$ 59,732

Stock-based Compensation - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Oct. 11, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 74,000	$ 88,000	$ 224,000	$ 236,000
Unrecognized stock-based compensation expense related to stock options	500,000		500,000
Unrecognized stock-based compensation expense related to stock-based awards	0		$ 0
Subsequent Event
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Unrecognized stock-based compensation expense related to stock-based awards					$ 2,500,000
Stock Options
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Unrecognized stock-based compensation expense, expected period for recognition			2 years 1 month 7 days
RSUs
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Unrecognized stock-based compensation expense, expected period for recognition			3 years 1 month 7 days
Unrecognized stock-based compensation expense related to stock-based awards	$ 10,100,000		$ 10,100,000

Stock-based Compensation - Summary of Allocation of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 74	$ 88	$ 224	$ 236
Cost of Revenue
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	6	6	16	14
Sales and Marketing Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	35	43	108	119
Product, Technology, and Development Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	24	28	72	76
General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 9	$ 11	$ 28	$ 27

Earnings Per Share - Additional Information (Details)	9 Months Ended
Earnings Per Share - Additional Information (Details)	Sep. 30, 2017 Vote shares
Earnings Per Share Basic [Line Items]
Conversion of stock, description	Each share of Class B common stock was and is convertible into one share of Class A common stock at the option of the holder at any time. In addition, each share of Class B common stock was and is automatically convertible into one share of Class A common stock upon any transfer of such share, which is defined to include entering into a voting agreement, whether or not for value, except for certain transfers described in both the Company’s Third Amended and Restated Certificate of Incorporation and Fourth Amended and Restated Certificate of Incorporation, including, without limitation, transfers to certain family members of the transferor stockholder. Upon either the death or voluntary termination of the Company’s Chief Executive Officer, all shares of Class B common stock were and are automatically convertible into one share of Class A common stock.
Class A Common Stock
Earnings Per Share Basic [Line Items]
Right to voting	one (1) vote for each share
Number of votes entitled to stockholders per share	1
Class B Common Stock
Earnings Per Share Basic [Line Items]
Right to voting	ten votes for each share
Number of votes entitled to stockholders per share	10
Class of share converted to another class	one share of Class A common stock
Conversion of stock \| shares	1

Earnings Per Share - Schedule of Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net (Loss) Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income	$ 2,379	$ 2,138	$ 10,932	$ 2,659
Net income attributable to participating securities	(1,401)	(1,260)	(6,446)	(1,554)
Net income attributable to common stockholders — basic	978	878	4,486	1,105
Net income attributable to participating securities	(1,345)	(1,222)	(6,198)	(1,507)
Net income attributable to common stockholders — diluted	$ 1,034	$ 916	$ 4,734	$ 1,152
Weighted-average number of shares of common stock used in computing net income per share attributable to common stockholders:
Weighted-average number of shares of common stock used in computing net income per share attributable to common stockholders — basic	42,262,035	44,692,419	42,168,904	44,665,063
Dilutive effect of share equivalents resulting from stock options	4,305,138	3,376,954	4,141,726	3,375,691
Weighted-average number of shares of common stock used in computing net income per share — diluted	46,567,173	48,069,373	46,310,630	48,040,754
Net income per share attributable to common stockholders: (Note 10)
Basic	$ 0.02	$ 0.02	$ 0.11	$ 0.02
Diluted	$ 0.02	$ 0.02	$ 0.10	$ 0.02

Earnings Per Share - Schedule of Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Weighted-average Shares Outstanding (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Stock Options Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive common stock equivalents excluded from calculation of diluted weighted-average shares outstanding		1,332,666	816,318	1,226,182
Restricted Stock Units Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive common stock equivalents excluded from calculation of diluted weighted-average shares outstanding	2,353,152	1,124,694	2,353,152	1,124,694

Segment and Geographic Information - Summary of Operations by Segment and Geographical Area (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment revenue:
Revenue	$ 82,989	$ 53,136	$ 226,264	$ 137,377
Segment (loss) income from operations:
Total income from operations	2,863	3,257	15,242	3,965
United States
Segment revenue:
Revenue	80,394	52,435	219,954	136,195
Segment (loss) income from operations:
Total income from operations	9,337	8,351	33,617	16,818
International
Segment revenue:
Revenue	2,595	701	6,310	1,182
Segment (loss) income from operations:
Total income from operations	$ (6,474)	$ (5,094)	$ (18,375)	$ (12,853)

Subsequent Events - Additional Information (Details) - USD ($)				1 Months Ended	9 Months Ended
	Oct. 25, 2017	Oct. 16, 2017	Aug. 23, 2016	Oct. 31, 2017	Sep. 30, 2017	Jun. 21, 2017	Dec. 31, 2016
Class A Common Stock
Subsequent Event [Line Items]
Convertible preferred stock, terms of conversion					in connection with the closing of the Company’s IPO, each share of Preferred Stock was converted into approximately six shares of Class A common stock pursuant to the terms of the Third Amended and Restated Certificate of Incorporation.
Common stock, shares authorized					120,020,700	120,020,700	120,020,700
Common stock, par value					$ 0.001	$ 0.001	$ 0.001
Class B Common Stock
Subsequent Event [Line Items]
Common stock, shares authorized					80,013,800	80,013,800	80,013,800
Common stock, par value					$ 0.001	$ 0.001	$ 0.001
Series E Preferred Stock
Subsequent Event [Line Items]
Stock issuance costs			$ 268,000
IPO
Subsequent Event [Line Items]
Convertible preferred stock, terms of conversion					Also, in connection with the closing of the IPO, each share of Series A Preferred Stock automatically converted into 6.0000023 shares of common stock, each share of Series B Preferred Stock converted into 6.0000015 shares of common stock, each share of Series C Preferred Stock converted into 6 shares of common stock, each share of Series D Preferred Stock converted into 6.0000159 shares of common stock and each share of Series E Preferred Stock converted into 6 shares of common stock, one-third of which number of shares of common stock was Class A common stock and two thirds of which number of shares of common stock was Class B common stock. Each such share of Class B common stock issued upon conversion of the Preferred Stock was then immediately converted into one share of Class A common stock.
Subsequent Event
Subsequent Event [Line Items]
Conversion of convertible securities into shares of common stock		9,091,484
Preferred stock, shares outstanding		0
Preferred stock, shares authorized		10,000,000
Preferred stock, par value		$ 0.001
Subsequent Event \| RSUs
Subsequent Event [Line Items]
RSUs granted	762,704
RSUs grant date fair value per share	$ 29.01
RSUs granted, aggregate value	$ 22,100,000
Subsequent Event \| Amended and Restated 2015 Equity Incentive Plan
Subsequent Event [Line Items]
Additional shares granted under plan				0
Subsequent Event \| Class A Common Stock
Subsequent Event [Line Items]
Conversion of convertible securities into shares of common stock		60,564,678
Convertible preferred stock converted into common stock		6
Common stock, shares authorized		500,000,000
Common stock, par value		$ 0.001
Subsequent Event \| Class A Common Stock \| Selling Shareholders
Subsequent Event [Line Items]
Number of shares issued and sold		7,605,000
Subsequent Event \| Class A Common Stock \| Subsequent Conversion
Subsequent Event [Line Items]
Conversion of convertible securities into shares of common stock		40,376,452
Subsequent Event \| Class B Common Stock
Subsequent Event [Line Items]
Conversion of convertible securities into shares of common stock		40,376,452
Common stock, shares authorized		100,000,000
Common stock, par value		$ 0.001
Subsequent Event \| IPO
Subsequent Event [Line Items]
Preferred stock, shares outstanding		0
Subsequent Event \| IPO \| Allen And Company LLC and Associated Persons
Subsequent Event [Line Items]
Percentage of ownership of outstanding preferred stock		13.50%
Subsequent Event \| IPO \| Class A Common Stock
Subsequent Event [Line Items]
Number of shares issued and sold		3,205,000
Number of shares issued for services		705,000
Offering price per share		$ 16.00
Aggregate gross proceeds		$ 51,300,000
Net proceeds received after deducting underwriting discounts and commissions and offering costs		43,000,000
Underwriting discounts and commissions		3,600,000
Stock issuance costs		$ 4,700,000
Conversion of convertible securities into shares of common stock		20,188,226
Subsequent Event \| IPO \| Class A Common Stock \| Allen And Company LLC and Associated Persons
Subsequent Event [Line Items]
Number of shares issued for services		2,190,200
Offering price per share		$ 14.88
Aggregate gross proceeds		$ 32,590,176
Underwriting discounts and commissions		$ 2,453,024
Subsequent Event \| IPO \| Series A Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock converted into common stock		6.0000023
Subsequent Event \| IPO \| Series B Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock converted into common stock		6.0000015
Subsequent Event \| IPO \| Series C Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock converted into common stock		6
Subsequent Event \| IPO \| Series D Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock converted into common stock		6.0000159
Subsequent Event \| IPO \| Series E Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock converted into common stock		6

Segment and Geographic Information - Additional Information (Details)	9 Months Ended
	Sep. 30, 2017 Segment
Segment Reporting [Abstract]
Number of reportable segments	2

Organization and Business Description - Additional Information (Details)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
State of incorporation	Delaware
Date of incorporation	Jun. 26, 2015