Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Feb. 23, 2018	Jun. 30, 2017
Document And Entity Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Entity Registrant Name	CarGurus, Inc.
Trading Symbol	CARG
Entity Central Index Key	0001494259
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Public Float			$ 771,446,882
Class A Common Stock
Document And Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		77,890,576
Class B Common Stock
Document And Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		28,235,290

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 87,709	$ 29,476
Investments	50,000	44,774
Accounts receivable, net of allowance for doubtful accounts of $494 and $164, respectively	12,577	6,653
Prepaid income taxes	1,533	1,815
Prepaid expenses and other current assets	5,385	2,789
Total current assets	157,204	85,507
Property and equipment, net	16,563	12,780
Restricted cash	1,843	2,044
Deferred tax assets	825
Other long–term assets	159
Total assets	176,594	100,331
Current liabilities
Accounts payable	23,908	16,426
Accrued expenses	13,588	8,384
Deferred revenue	4,305	3,330
Deferred rent	1,165	910
Total current liabilities	42,966	29,050
Deferred rent, net of current portion	5,648	5,673
Deferred tax liabilities		292
Other non–current liabilities	955	590
Total liabilities	49,569	35,605
Commitments and contingencies (Note 6)
Convertible preferred stock		132,698
Stockholders’ equity:
Additional paid–in capital	185,190	3,714
Accumulated deficit	(58,499)	(71,698)
Accumulated other comprehensive income (loss)	228	(30)
Total stockholders’ equity (deficit)	127,025	(67,972)
Total liabilities, convertible preferred stock, and stockholders’ equity (deficit)	176,594	100,331
Class A Common Stock
Stockholders’ equity:
Common stock	78	14
Class B Common Stock
Stockholders’ equity:
Common stock	$ 28	$ 28

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts	$ 494	$ 164
Class A Common Stock
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	77,884,754	14,022,132
Common stock, shares outstanding	77,884,754	14,022,132
Class B Common Stock
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	28,226,104	28,044,264
Common stock, shares outstanding	28,226,104	28,044,264

Consolidated Statements of Operations - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Income Statement [Abstract]

Revenue

$ 316,861

$ 198,141

$ 98,588

Cost of revenue

^[1]

17,609

9,575

4,234

Gross profit

299,252

188,566

94,354

Operating expenses:

Sales and marketing

236,165

154,125

81,877

Product, technology, and development

22,470

11,453

8,235

General and administrative

22,688

12,783

5,801

Depreciation and amortization

2,655

1,634

969

Total operating expenses

283,978

179,995

96,882

Income (loss) from operations

15,274

8,571

(2,528)

Other income (expense), net

563

374

(12)

Income (loss) before income taxes

15,837

8,945

(2,540)

Provision for (benefit from) income taxes

2,638

2,448

(904)

Net income (loss)

13,199

6,497

(1,636)

Reconciliation of net income (loss) to net income (loss) attributable to common stockholders:

Net income (loss)

13,199

6,497

(1,636)

Deemed dividend to preferred stockholders

(32,087)

(15,930)

Net income attributable to participating securities

(6,098)

Net income (loss) attributable to common stockholders — basic

7,101

(25,590)

(17,566)

Net income (loss)

13,199

6,497

(1,636)

Deemed dividend to preferred stockholders

(32,087)

(15,930)

Net income attributable to participating securities

(5,829)

Net income (loss) attributable to common stockholders — diluted

$ 7,370

$ (25,590)

$ (17,566)

Net income (loss) per share attributable to common stockholders: (Note 9)

Basic

$ 0.13

$ (0.58)

$ (0.41)

Diluted

$ 0.12

$ (0.58)

$ (0.41)

Weighted–average number of shares of common stock used in computing net income (loss) per share attributable to common stockholders:

Basic

55,835,265

44,138,922

43,141,236

Diluted

60,637,584

44,138,922

43,141,236

[1]	Includes depreciation and amortization expense for the years ended December 31, 2017, 2016, and 2015 of $1,140, $438, and $153, respectively.

Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 13,199	$ 6,497	$ (1,636)
Other comprehensive income (loss):
Foreign currency translation adjustment	258	(30)
Comprehensive income (loss)	$ 13,457	$ 6,467	$ (1,636)

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Preferred Stock	Series B Preferred Stock	Series C Preferred Stock	Series D Preferred Stock	Series E Preferred Stock	Member Units	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Retained Earnings (Accumulated Deficit)
Beginning balance at Dec. 31, 2014	$ 7,878						$ 5,864			$ 175		$ 1,839
Beginning balance, Shares at Dec. 31, 2014							14,764,149
Issuance of member units upon exercise of unit options	59						$ 59
Issuance of member units upon exercise of unit options, Shares							1,017,583
Conversion from LLC to Corporation	(5,750)							$ 15	$ 30	1,185		(1,057)
Convertible preferred stock, Conversion from LLC to Corporation, Shares		3,333,000	3,329,497	1,648,978
Convertible preferred stock, Conversion from LLC to Corporation		$ 1,750	$ 2,600	$ 1,400
Member units, Conversion from LLC to Corporation, Shares							(15,781,732)
Member units, Conversion from LLC to Corporation							$ (5,923)
Conversion from LLC to Corporation, Shares								14,940,514	29,881,028
Net income (loss)	(1,636)											(1,636)
Convertible preferred stock, Issuance of preferred stock, net of issuance costs, Shares					1,673,105
Convertible preferred stock, Issuance of preferred stock, net of issuance costs					$ 67,872
Repurchase of stock	(17,756)	$ (149)	$ (26)	$ (69)								(17,756)
Repurchase of stock, Shares		(283,394)	(33,443)	(81,123)				(64,556)	(129,112)
Issuance of common stock upon exercise of stock options	8									8
Issuance of common stock upon exercise of stock options, Shares								3,996	7,992
Tax benefit related to exercise of stock options	26									26
Stock–based compensation expense	1,040									1,040
Ending balance at Dec. 31, 2015	(16,131)							$ 15	$ 30	2,434		(18,610)
Convertible preferred stock, Ending balance, Shares at Dec. 31, 2015		3,049,606	3,296,054	1,567,855	1,673,105
Convertible preferred stock, Ending balance at Dec. 31, 2015		$ 1,601	$ 2,574	$ 1,331	$ 67,872
Ending balance, Shares at Dec. 31, 2015								14,879,954	29,759,908
Net income (loss)	6,497											6,497
Convertible preferred stock, Issuance of preferred stock, net of issuance costs, Shares						1,107,202
Convertible preferred stock, Issuance of preferred stock, net of issuance costs						$ 59,732
Repurchase of stock	(59,588)	$ (118)	$ (279)	$ (15)				$ (1)	$ (2)			(59,585)
Repurchase of stock, Shares		(224,903)	(357,568)	(17,243)				(899,046)	(1,798,092)
Issuance of common stock upon exercise of stock options and vesting of restricted stock units	137									137
Issuance of common stock upon exercise of stock options and vesting of restricted stock units, Shares								41,224	82,448
Tax benefit related to exercise of stock options	821									821
Stock–based compensation expense	322									322
Foreign currency translation adjustment	(30)										$ (30)
Ending balance at Dec. 31, 2016	(67,972)							$ 14	$ 28	3,714	(30)	(71,698)
Convertible preferred stock, Ending balance, Shares at Dec. 31, 2016		2,824,703	2,938,486	1,550,612	1,673,105	1,107,202
Convertible preferred stock, Ending balance at Dec. 31, 2016	132,698	$ 1,483	$ 2,295	$ 1,316	$ 67,872	$ 59,732
Ending balance, Shares at Dec. 31, 2016								14,022,132	28,044,264
Net income (loss)	13,199											13,199
Issuance of stock, net of issuance/offering costs	43,240							$ 3		43,237
Issuance of stock, net of issuance/offering costs, Shares								3,205,000
Issuance of common stock upon exercise of stock options	398									398
Issuance of common stock upon exercise of stock options, Shares								92,944	181,840
Stock–based compensation expense	5,204									5,204
Conversion of preferred stock	132,698							$ 61		132,637
Convertible preferred stock, Conversion of preferred stock, Shares		(2,824,703)	(2,938,486)	(1,550,612)	(1,673,105)	(1,107,202)
Convertible preferred stock, Conversion of preferred stock		$ (1,483)	$ (2,295)	$ (1,316)	$ (67,872)	$ (59,732)
Conversion of preferred stock, Shares								60,564,678
Foreign currency translation adjustment	258										258
Ending balance at Dec. 31, 2017	$ 127,025							$ 78	$ 28	$ 185,190	$ 228	$ (58,499)
Ending balance, Shares at Dec. 31, 2017								77,884,754	28,226,104

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities
Net income (loss)	$ 13,199	$ 6,497	$ (1,636)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	3,795	2,072	1,122
Unrealized currency loss on foreign denominated transactions	128
Deferred taxes	(1,117)	782	(649)
Provision for doubtful accounts	1,117	508	284
Stock–based compensation expense	5,028	322	1,040
Excess tax benefit related to exercise of stock options		(821)	(26)
Changes in operating assets and liabilities:
Accounts receivable, net	(7,039)	(1,432)	(716)
Prepaid expenses, prepaid income taxes, and other assets	(2,287)	(2,226)	(820)
Accounts payable	6,244	5,811	6,104
Accrued expenses	5,191	4,118	2,469
Deferred revenue	962	1,856	1,089
Deferred rent	227	1,927	4,654
Other non–current liabilities	243	590
Net cash provided by operating activities	25,691	20,004	12,915
Investing Activities
Purchases of property and equipment	(5,157)	(5,846)	(6,353)
Capitalization of website development costs	(2,215)	(1,372)	(1,262)
Investments in certificates of deposit	(50,000)	(59,774)
Maturities of certificates of deposit	44,774	15,000
Net cash used in investing activities	(12,598)	(51,992)	(7,615)
Financing Activities
Initial public offering proceeds, net of offering costs paid of $3,308	44,382
Proceeds from issuance of preferred stock, net of offering costs		59,732	67,872
Proceeds from exercise of unit options and stock options	398	137	67
Excess tax benefit related to exercise of stock options		821	26
Cash paid for repurchase of preferred stock, common stock, and vested options		(60,000)	(18,000)
Net cash provided by financing activities	44,780	690	49,965
Impact of foreign currency on cash, cash equivalents, and restricted cash	159	(45)
Net increase (decrease) in cash, cash equivalents, and restricted cash	58,032	(31,343)	55,265
Cash, cash equivalents, and restricted cash at beginning of period	31,520	62,863	7,598
Cash, cash equivalents, and restricted cash at end of period	89,552	31,520	62,863
Supplemental disclosure of cash flow information:
Cash paid for income taxes	4,393	2,045	316
Cash paid for interest	29	26	$ 17
Supplemental disclosure of non–cash investing and financing activities:
Unpaid purchases of property and equipment	510	$ 476
Unpaid initial public offering costs	1,142
Capitalized stockholders' compensation in website development costs	$ 176

Organization and Business Description	12 Months Ended
Organization and Business Description	Dec. 31, 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 and dealer reputation, as well as 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. Prior to June 26, 2015, the Company operated as CarGurus LLC and was organized on November 10, 2005 as a limited liability company under the laws of the Commonwealth of Massachusetts. In connection with the conversion into a Delaware corporation, the Class A unitholders received an equal number of shares of Class B common stock, the Class B unitholders received an equal number of shares of Series A convertible preferred stock, or Series A Preferred Stock, the Class C unitholders received an equal number of shares of Series B convertible preferred stock, or Series B Preferred Stock, and the Class D unitholders received an equal number of shares of Series C convertible preferred stock, or Series C Preferred Stock. In connection with this conversion, the Company also reclassified members' retained earnings of $1,057, accumulated under CarGurus LLC, to additional paid-in capital of CarGurus, Inc. On October 16, 2017, the Company completed its initial public offering (“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 $43.2 million in net proceeds after deducting $3.6 million of underwriting discounts and commissions and $4.5 million in offering costs. In addition to shares of Class A common stock issued and sold by the Company, certain selling stockholders 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. The Company is subject to a number of risks and uncertainties common to companies in its and 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

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

The accompanying consolidated financial statements reflect the application of certain significant accounting policies as described below and elsewhere in these notes to the consolidated financial statements. The Company believes that a significant accounting policy is one that is both important to the portrayal of the Company’s financial condition and results, and requires management’s most difficult, subjective, or complex judgments, often as the result of the need to make estimates about the effect of matters that are inherently uncertain.

Basis of Presentation

The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America, or GAAP. Any reference in these notes to applicable guidance is meant to refer to the authoritative U.S. generally accepted accounting principles as found in the Accounting Standards Codification, or ASC, and Accounting Standards Update, or ASU, of the Financial Accounting Standards Board, or FASB.

Principles of Consolidation

The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All material intercompany accounts and transactions have been eliminated in consolidation.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosures of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenue and expenses during the reporting period.

Significant estimates relied upon in preparing these consolidated financial statements 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 prior to the IPO, stock‑based compensation expense, and the recoverability of the Company’s net deferred tax assets and related valuation allowance.

Although the Company regularly assesses these estimates, actual results could differ materially from these estimates. Changes in estimates are recorded in the period in which they become known. The Company bases its estimates on historical experience and various other assumptions that it believes to be reasonable under the circumstances. Actual results may differ from management’s estimates if these results differ from historical experience, or other assumptions do not turn out to be substantially accurate, even if such assumptions are reasonable when made.

Subsequent Events 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 relative to 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.

Revenue Recognition

The Company derives its revenue from two primary sources: marketplace subscription revenue, which consists of listing and display advertising subscriptions from dealers, and advertising and other revenue, which consists primarily of display advertising revenue from auto manufacturers and other auto‑related brand advertisers.

The Company recognizes revenue when all of the following conditions are satisfied: (1) there is persuasive evidence of an arrangement; (2) the service has been provided to the customer; (3) the collection of fees is reasonably assured; and (4) the amount of fees to be paid by the customer is fixed or determinable.

The Company offers two types of paid marketplace listing products to dealers, Enhanced and Featured Listings, which require a contractual subscription with initial terms ranging from one month to one year. Contracts for customers generally auto‑renew on a monthly basis and are cancellable by dealers with 30‑days’ advance notice at the end of current term. In addition, the arrangement allows the dealers to access a dashboard to track sales leads and manage its account. Customers do not have the right to take possession of the Company’s software. The Company recognizes revenue in accordance with ASC 605, Revenue Recognition. The Company recognizes revenue on a monthly basis as revenue is earned. These contracts generally provide the customer with the ability to list an unlimited amount of automobile inventory on the Company’s website.

In addition to listing dealers’ inventory on its marketplace, the Company periodically enters into multiple‑element service arrangements that provide dealers with Enhanced or Featured Listing products, as well as other advertising and customer acquisition products including display advertising, which appears on its marketplace and on other sites on the internet and requires a paid subscription under contracts with initial terms ranging from one month to one year. Contracts for customers generally auto‑renew on a monthly basis and are cancellable by dealers with 30‑days’ advance notice at the end of the current term.

The Company assesses arrangements with multiple deliverables under ASU No. 2009‑13, Revenue Recognition (Topic 605), Multiple‑Deliverable Revenue Arrangements — a Consensus of the FASB Emerging Issues Task Force. Pursuant to ASU 2009‑13, in order to treat deliverables in a multiple‑element arrangement as separate units of accounting, the deliverables must have stand‑alone value upon delivery. If the deliverables have stand‑alone value upon delivery, the Company accounts for each deliverable separately. The Company has concluded that each element in the arrangement has stand‑alone value as the individual services can be sold separately. In addition, there is no right of refund once a service has been delivered. Therefore, the Company has concluded each element of the arrangement is a separate unit of accounting. While these arrangements are considered multiple element‑arrangements, the recognition of the units of accounting follow a consistent ratable recognition given the pattern over which services are provided.

Advertising and other revenue consists primarily of non‑dealer display advertising revenue from auto manufacturers and other auto‑related brand advertisers sold on a cost per thousand impressions, or CPM, basis. Impressions are the number of times an advertisement is loaded on a web page. Pricing is primarily based on advertisement size and position on the Company’s mobile applications and websites, and fees are generally billed monthly. The Company recognizes such revenue as impressions are delivered.

The Company does not provide minimum impression guarantees or other types of minimum guarantees in its contracts with customers.

The Company sells advertising directly to auto manufacturers and other auto‑related brand advertisers, as well as indirectly through revenue sharing arrangements with advertising exchange partners. Company‑sold advertising is not subject to revenue sharing arrangements. Company‑sold advertising revenue is recognized based on the gross amount charged to the advertiser. Partner‑sold advertising revenue is recognized based on the net amount of revenue received from the content partners.

Revenue from advertising sold directly by the Company to auto manufacturers and other auto‑related brand advertisers is recorded on a gross basis predominately because the Company is the primary obligor responsible for fulfilling advertisement delivery, including the acceptability of the services delivered. The Company enters into contractual arrangements directly with advertisers and is directly responsible for the fulfillment of the contractual terms and any remedy for issues with such fulfillment. The Company also has latitude in establishing the selling price with the advertiser, as the Company sells advertisements at a rate determined at its sole discretion.

Advertising revenue subject to revenue sharing agreements between the Company and advertising exchange partners is recognized based on the net amount of revenue received from the partner predominately because the advertising partner, and not the Company, is the primary obligor responsible for fulfillment, including the acceptability of the services delivered. In partner‑sold advertising arrangements, the advertising partner has a direct contractual relationship with the advertiser. There is no contractual relationship between the Company and the advertiser for partner‑sold transactions. When an advertising exchange partner sells advertisements, the partner is responsible for fulfilling the advertisements, and accordingly, the Company has determined the advertising partner is the primary obligor. Additionally, the Company does not have any latitude in establishing the price with the advertiser for partner‑sold advertising.

Revenue is presented net of any taxes collected from customers.

The Company establishes sales allowances at the time of revenue recognition based on its history of adjustments and credits provided to its customers. Sales allowances relate primarily to credits issued for service interruption. In assessing the adequacy of the sales allowance, the Company evaluates its history of adjustments and credits made through the date of the issuance of the financial statements. Estimated sales adjustments and credits and ultimate losses may vary from actual results which could be material to the financial statements; however, to date, actual sales allowances have been materially consistent with the Company’s estimates. Sales allowances are recorded as a reduction to revenue in the consolidated statements of operations.

Deferred Revenue

Deferred revenue primarily consists of payments received in advance of revenue recognition from the Company’s marketplace revenue and is recognized as the revenue recognition criteria are met. The Company generally invoices its customers monthly. Accordingly, the deferred revenue balances do not represent the total contract value of annual or multiyear subscription agreements. Deferred revenue that is expected to be recognized during the succeeding 12‑month period is recorded as current deferred revenue and the remaining portion is recorded as noncurrent in the consolidated balance sheets. All deferred revenue was recorded as current for all periods presented.

Cost of Revenue

Cost of revenue primarily consists of costs related to supporting and hosting the Company’s website and product offerings. These costs include salaries, benefits, incentive compensation and stock‑based compensation expense related to the customer support team, and third‑party service provider costs such as data center and networking expenses, allocated overhead, depreciation and amortization expense associated with the Company’s property and equipment, and amortization of capitalized website development costs.

Concentration of Credit Risk and Significant Customers

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 years ended December 31, 2017 and 2016, no individual customer accounted for more than 10% of total revenue. For the year ended December 31, 2015, one customer accounted for 14% of total revenue.

As of December 31, 2017, two customers accounted for 29% and 17% 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 December 31, 2017 or 2016.

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.

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 Company’s investment policy, which was approved by the Audit Committee of the Company’s board of directors, or the Board, permits investments in fixed income securities, including U.S. government and agency securities, non‑U.S. government securities, money market instruments, commercial paper, certificates of deposit, corporate bonds, and asset‑backed securities.

As of December 31, 2017 and 2016, investments consisted of U.S. certificates of deposit, or CDs, with remaining maturities of less than twelve months. The Company classifies CDs with readily determinable market values as held‑to‑maturity, because it is the Company’s intention to hold such investments until they mature. As such, investments were recorded at amortized cost at December 31, 2017 and 2016. The Company adjusts the cost of investments for amortization of premiums and accretion of discounts to maturity. The Company includes such amortization and accretion in interest income (expense).

Realized gains and losses from sales of the Company’s investments are included in other income (expense), net. There were no realized gains or losses on investments for the years ended December 31, 2017, 2016 or 2015.

The Company reviews investments for other‑than‑temporary impairment whenever the fair value of an investment is less than the amortized cost and evidence indicates that an investment’s carrying amount is not recoverable within a reasonable period of time. Other‑than‑temporary impairments of investments are recognized in the consolidated statements of operations if the Company has experienced a credit loss, has the intent to sell the investment, or if it is more likely than not that the Company will be required to sell the investment before recovery of the amortized cost basis. Evidence considered in this assessment includes reasons for the impairment, compliance with the Company’s investment policy, the severity and duration of the impairment, and changes in value subsequent to the end of the period. As of December 31, 2017 and 2016, the Company determined that no other‑than‑temporary impairments were required to be recognized in the consolidated statements of operations.

Restricted Cash

At December 31, 2017 and 2016, restricted cash was $1,843 and $2,044, respectively, and primarily related to cash held at a financial institution in an interest‑bearing cash account as collateral for two letters of credit related to the contractual provisions for the Company’s building lease security deposits. As of December 31, 2017 and 2016, the restricted cash is classified as a long‑term asset.

Accounts Receivable and Allowance for Doubtful Accounts

Accounts receivable are recorded based on the amount due from the customer and do not generally bear interest. The Company offsets gross trade accounts receivable with an allowance for doubtful accounts. The allowance for doubtful accounts is the Company’s best estimate of the amount of probable credit losses in the Company’s existing accounts receivable and is based upon historical loss patterns, the number of days that billings are past due, and an evaluation of the potential risk of loss associated with specific accounts. Account balances are charged against the allowance after all means of collection have been exhausted and the potential for recovery is considered remote. The Company does not have any off‑balance sheet credit exposure related to its customers. Provisions for allowances for doubtful accounts are recorded in general and administrative expense.

The Company considers current economic trends when evaluating the adequacy of the allowance for doubtful accounts. If circumstances relating to specific customers change, or unanticipated changes occur in the general business environment, particularly as it affects auto dealers, the Company’s estimates of the recoverability of receivables could be further adjusted.

Below is a summary of the changes in the Company’s allowance for doubtful accounts for the years ended December 31, 2017, 2016, and 2015:

	Balance at Beginning of Period		Provision		Write–offs, net of recoveries			Balance at End of Period
Year ended December 31, 2017	$	164	$	1,117	$	(787	)	$	494
Year ended December 31, 2016		75		508		(419	)		164
Year ended December 31, 2015		30		284		(239	)		75

Property and Equipment

Property and equipment are stated at cost less accumulated depreciation and amortization using the straight‑line method over the estimated useful lives of the assets. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. The estimated useful lives of the Company’s property and equipment are as follows:

	Estimated Useful Life (In Years)
Computer equipment	3
Capitalized software	3
Website development costs	3
Furniture and fixtures	5
Leasehold improvements	Lesser of asset life or lease term

Expenditures for maintenance and repairs are charged to expense as incurred, whereas major betterments are capitalized as additions to property and equipment. The Company reviews its property and equipment whenever events or changes in circumstances indicate that the carrying value of certain assets might not be recoverable. In these instances, the Company recognizes an impairment loss when it is probable that the estimated cash flows are less than the carrying value of the asset.

Foreign Currency Translation

The reporting currency of the Company is the U.S. dollar. The functional currency of the Company’s foreign subsidiaries is the local currency of each subsidiary. All assets and liabilities in the balance sheets of entities whose functional currency is a currency other than the U.S. dollar are translated into U.S. dollar equivalents at exchange rates as follows: (1) asset and liability accounts at period‑end rates; (2) income statement accounts at weighted‑average exchange rates for the period; and (3) stockholders’ equity accounts at historical exchange rates. The resulting translation adjustments are excluded from income (loss) and reflected as a separate component of stockholders’ equity (deficit). Foreign currency transaction gains and losses are included in net income (loss) for the period. The Company may periodically have certain intercompany foreign currency transactions that are deemed to be of a long‑term investment nature; exchange adjustments related to those transactions are made directly to a separate component of stockholders’ equity (deficit).

Capitalized Website and Software Development Costs

The Company capitalizes certain costs associated with the development of its websites and internal‑use software products after the preliminary project stage is complete, and until the software is ready for its intended use. Research and development costs incurred during the preliminary project stage or costs incurred for data conversion activities, training, maintenance, and general and administrative or overhead costs are expensed as incurred. Capitalization begins when the preliminary project stage is complete; management authorizes and commits to the funding of the software project with appropriate authority; it is probable the project will be completed; the software will be used to perform the functions intended; and certain functional and quality standards have been met. Qualified costs incurred during the operating stage of the Company’s software applications relating to upgrades and enhancements are capitalized to the extent it is probable that they will result in added functionality, while costs that cannot be separated between maintenance of, and minor upgrades and enhancements to, internal‑use software are expensed as incurred.

Capitalized website development costs and software development costs are amortized on a straight‑line basis over their estimated useful life of three years. Management evaluates the useful lives of these assets on an annual basis and tests for impairment whenever events or changes in circumstances occur that could impact the recoverability of these assets.

During the years ended December 31, 2017, 2016, and 2015, the Company capitalized $2,215, $1,372, and $1,262 of software and website development costs, respectively. The Company recorded amortization expense associated with its capitalized software and website development costs of $812, $343, and $153 for the years ended December 31, 2017, 2016, and 2015, respectively.

Impairment of Long‑Lived Assets

The Company evaluates the recoverability of long‑lived assets, such as property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. During this review, the Company re‑evaluates the significant assumptions used in determining the original cost and estimated lives of long‑lived assets. Although the assumptions may vary from asset to asset, they generally include operating results, changes in the use of the asset, cash flows, and other indicators of value. Management then determines whether the remaining useful life continues to be appropriate, or whether there has been an impairment of long‑lived assets based primarily upon whether expected future undiscounted cash flows are sufficient to support the assets’ recovery. Recoverability of these assets is measured by comparison of the carrying amount of the asset to the future undiscounted cash flows the asset is expected to generate. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset.

For the years ended December 31, 2017, 2016, and 2015, the Company did not identify any impairment of its long‑lived assets.

Income Taxes

The Company accounts for income taxes in accordance with the asset and liability method. Under this method, deferred tax assets and liabilities are recognized based on temporary differences between the financial reporting and income tax bases of assets and liabilities using statutory rates. In addition, this method requires a valuation allowance against net deferred tax assets if, based upon the available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.

The Company accounts for uncertain tax positions recognized in the consolidated financial statements by prescribing a more‑likely‑than‑not threshold for financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. The Company has no recorded liabilities for uncertain tax positions as of December 31, 2017 and 2016.

Disclosure of Fair Value of Financial Instruments

The carrying amounts of the Company’s financial instruments, which include cash and cash equivalents, investments, accounts receivable, accounts payable, and accrued expenses, approximated their fair values at December 31, 2017 and 2016 due to the short‑term nature of these instruments.

The Company has evaluated the estimated fair value of financial instruments using available market information. The use of different market assumptions, estimation methodologies, or both, could have a significant effect on the estimated fair value amounts. See Note 3 for further discussion.

Stock‑Based Compensation

For stock‑based awards issued under the Company’s stock‑based compensation plans, which are more fully described in Note 8, the fair value of each award is estimated on the date of grant, and, up through the year ended December 31, 2016, an estimated forfeiture rate was used when calculating stock‑based compensation expense for the period. The Company recognizes compensation expense for service-based awards on a straight-line basis over the requisite service period for each separate vesting portion of the award, with the amount of compensation expense recognized at any date at least equaling the portion of the grant-date fair value of the award that is vested at that date.

Certain awards granted by the Company prior to the IPO were subject to service‑based vesting conditions and a performance‑based vesting condition achieved upon a liquidity event, defined as either a change of control or an IPO. 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. Upon the achievement of the liquidity event, the Company recorded previously unrecognized cumulative stock-based compensation expense of $2.5 million related to these awards. Although the performance based vesting condition was satisfied, under the terms of the awards, the settlement of such vested RSUs and the issuance of common stock with respect to such vested RSUs, will occur on April 10, 2018, one hundred eighty-one days after the satisfaction of the performance condition.

Given the absence of an active market for the Company’s common stock prior to the IPO, the Board, the members of which the Company believes have extensive business, finance, and venture capital experience, was required to estimate the fair value of the Company’s common stock at the time of each grant of a stock‑based award. The Company and the Board utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants’ Technical Practice Aid, Valuation of Privately‑Held Company Equity Securities Issued as Compensation, to estimate the fair value of its common stock. Each valuation methodology includes estimates and assumptions that require the Company’s judgment. These estimates and assumptions include a number of objective and subjective factors in determining the value of the Company’s common stock at each grant date, including the following factors: (1) prices paid for the Company’s convertible Preferred Stock, which the Company had sold to outside investors in arm’s‑length transactions, and the rights, preferences, and privileges of the Company’s convertible Preferred Stock and common stock; (2) valuations performed by an independent valuation specialist; (3) the Company’s stage of development and revenue growth; (4) the fact that the grants of stock‑based awards involved illiquid securities in a private company; and (5) the likelihood of achieving a liquidity event for the common stock underlying the stock‑based awards, such as an IPO or sale of the Company, given prevailing market conditions.

The Company believes this methodology was reasonable based upon the Company’s internal peer company analyses, and further supported by arm’s‑length transactions involving the Company’s convertible Preferred Stock. As the Company’s common stock was not actively traded, the determination of fair value involved assumptions, judgments, and estimates. If different assumptions had been made, stock‑based compensation expense, consolidated net income (loss), and consolidated net income (loss) per share could have been significantly different.

For RSUs granted subsequent to the IPO, the fair value is determined based on the closing price of the Company’s Class A common stock as reported on the Nasdaq Global Select Market on the date of grant.

For RSUs issued under the Company’s stock‑based compensation plans prior to the IPO, the fair value of each grant was calculated based on the estimated fair value of the Company’s common stock on the date of grant. The Company estimated the fair value of most stock option awards on the date of grant using the Black‑Scholes option‑pricing model. Certain stock option awards that have an exercise price that was materially above the current estimated fair market value of the Company’s stock are considered to be “deeply out of the money,” and are valued at the date of grant using a binomial lattice option‑pricing model.

The fair value of each option grant issued under the Company’s stock‑based compensation plans that was not considered “deeply out of the money,” was estimated using the Black‑Scholes option‑pricing model. As there was no public market for its common stock prior to the IPO, the Company determined the volatility for options granted based on an analysis of reported data for a peer group of companies that issued options with substantially similar terms. The expected volatility of granted options has been determined using a weighted‑average of the historical volatility measures of this peer group of companies. The expected life of options has been determined utilizing the “simplified method.” The simplified method is based on the average of the vesting tranches and the contractual life of each grant. The risk‑free interest rate is based on a treasury instrument whose term is consistent with the expected life of the stock options. The Company has not paid, and does not anticipate paying, cash dividends on its common stock; therefore, the expected dividend yield was assumed to be zero. In addition, the Company applied an estimated forfeiture rate of 5% in determining the expense recorded in the accompanying consolidated statements of operations for the years ended December 31, 2016 and 2015.

In March 2016, the FASB issued ASU 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 2017, the Company recorded tax benefits of $681, 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 consolidated statements of cash flows for the years ended December 31, 2016 or 2015. The adoption did not have any other material impact on the Company’s consolidated financial statements.

The weighted‑average fair value of options granted during the years ended December 31, 2016 and 2015 was $0.90 and $0.46, respectively. No options were granted during 2017. The weighted‑average assumptions utilized to determine the fair value of options granted are presented in the following table:


	2016			2015
Expected dividend yield		—			—
Expected volatility		49	%		64	%
Risk–free interest rate		1.57	%		1.73	%
Expected term (in years)		6.07			6.05

See Note 8 for a summary of the stock option and RSU activity for the years ended December 31, 2017, 2016, and 2015.

Advertising Costs

Advertising costs are expensed as incurred. Advertising expense, which is included within sales and marketing expense in the consolidated statements of operations, was $173,186, $112,167, and $61,865 for the years ended December 31, 2017, 2016, and 2015, respectively.

Leases

The Company categorizes leases at their inception as either operating or capital leases. On certain lease arrangements, the Company may receive rent holidays or other incentives. The Company recognizes lease costs on a straight‑line basis once control of the space is achieved, without regard to deferred payment terms, such as rent holidays, that defer the commencement date of required payments or escalating payment amounts. The difference between required lease payments and rent expense has been recorded as deferred rent. Additionally, incentives received are treated as a reduction of costs over the term of the agreement, as they are considered an inseparable part of the lease agreement.

Comprehensive Income (Loss)

Comprehensive income (loss) is defined as the change in stockholders’ equity of a business enterprise during a period from transactions and other events and circumstances from non‑owner sources. Comprehensive income (loss) consists of net income (loss) and other comprehensive (loss) income, which includes certain changes in equity that are excluded from net income (loss). Specifically, cumulative foreign currency translation adjustments are included in accumulated other comprehensive income (loss). As of December 31, 2017, 2016, and 2015, accumulated other comprehensive income (loss) is presented separately on the consolidated balance sheets and consists entirely of cumulative foreign currency translation adjustments.

Contingent Liabilities

The Company has certain contingent liabilities that arise in the ordinary course of business activities. The Company accrues for loss contingencies when losses become probable and are reasonably estimable. If the reasonable estimate of the loss is a range and no amount within the range is a better estimate, the minimum amount of the range is recorded as a liability. The Company does not accrue for contingent losses that, in its judgment, are considered to be reasonably possible, but not probable; however, it discloses the range of such reasonably possible losses.

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 the last day of the fiscal year following the fifth anniversary of the IPO 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, 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.

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.

Revenue Recognition

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 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. The Company currently expects to adopt the standard, using the modified retrospective method.

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. For all other entities the guidance in Update 2014-09 is effective for annual reporting periods beginning after December 15, 2018, and interim reporting periods within annual reporting periods beginning after December 15, 2019. As an emerging growth company, the Company expects to adopt the standard effective January 1, 2019; however, if the Company ceases to be an emerging growth company as of December 31, 2018, the Company will be required to adopt the standard in the fourth quarter of 2018.

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 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 the standard, 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.

Other Recent Accounting Pronouncements

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 all other 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.

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 all other 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.

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

12 Months Ended

Dec. 31, 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 December 31, 2017 and 2016:

	December 31, 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	$	60,709	$	—	$	—	$	60,709
Investments:
Certificates of deposit		—		50,000		—		50,000
Total	$	60,709	$	50,000	$	—	$	110,709

	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 years ended December 31, 2017 or 2016.

The following is a summary of cash, cash equivalents, and investments as of December 31, 2017 and 2016.

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
December 31, 2017:
Cash and cash equivalents due in 90 days or less	$	87,709	$	—	$	—	$	87,709
Investments:
Certificates of deposit due in one year or less		50,000						50,000
Total cash, cash equivalents, and investments	$	137,709	$	—	$	—	$	137,709

	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

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Property and Equipment, Net

4. Property and Equipment, Net

Property and equipment consists of the following:

	At December 31,
	2017			2016
Computer equipment	$	3,532		$	2,001
Capitalized software		174			114
Website development costs		4,895			2,680
Furniture and fixtures		4,421			3,386
Leasehold improvements		10,797			8,202
Construction in progress		46			119
		23,865			16,502
Less accumulated depreciation		(7,302	)		(3,722	)
Property and equipment, net	$	16,563		$	12,780

Depreciation and amortization expense, which includes amortization expense associated with capitalized software and website development costs, was $3,795, $2,072, and $1,122 for the years ended December 31, 2017, 2016, and 2015, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2017

Accrued Liabilities Current [Abstract]

Accrued Expenses

5. Accrued Expenses

Accrued expenses consist of the following:

	At December 31,
	2017		2016
Accrued bonuses	$	7,807	$	4,662
Other accrued expenses		5,781		3,722
	$	13,588	$	8,384

The Company had accrued bonuses of $7.8 million and $4.7 million at December 31, 2017 and 2016, respectively. The increase of $3.1 million in accrued bonuses is primarily due to increased headcount in 2017, as compared to 2016.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6. Commitments and Contingencies

Operating Leases

The Company leases its facilities under non‑cancelable operating leases with various expiration dates through January 2024. Rent expense for non-cancelable operating leases with free rental periods or scheduled rent increases is recognized on a straight-line basis over the terms of the leases. The difference between required lease payments and rent expense has been recorded as deferred rent.

As of December 31, 2017, the Company had deferred rent and rent incentives of $6,813, of which $1,165 and $5,648, respectively, are classified as a short‑term liability and a long‑term liability in the corresponding consolidated balance sheet. As of December 31, 2016, the Company had deferred rent and rent incentives of $6,583, of which $910 and $5,673, respectively, are classified as a short‑term liability and a long‑term liability in the corresponding consolidated balance sheet. Rent expense related to the operating leases for the years ended December 31, 2017, 2016, and 2015 was $5,994, $3,678, and $2,700 respectively.

Future minimum rental commitments under the Company’s operating leases at December 31, 2017 are as follows:

Year Ending December 31,	Operating Lease Commitments
2018	$	7,363
2019		7,653
2020		7,694
2021		7,794
2022 and thereafter		9,775
	$	40,279

Legal Matters

From time to time the Company may become involved in legal proceedings or be subject to claims arising in the ordinary course of its business. The Company is not presently subject to any pending or threatened litigation that it believes, if determined adversely to the Company, individually, or taken together, would reasonably be expected to have a material adverse effect on its business or financial results.

Guarantees and Indemnification Obligations

In the ordinary course of business, the Company enters into agreements with its customers that include commercial provisions with respect to licensing, infringement, indemnification, and other common 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 December 31, 2017, 2016, and 2015, the Company has not incurred any costs for guarantees or indemnities.

Convertible Preferred Stock and Stockholders' Equity

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Convertible Preferred Stock and Stockholders' Equity

7. Convertible Preferred Stock and Stockholders’ Equity

On July 7, 2015, the Company completed a Series D convertible preferred stock, or Series D Preferred Stock, offering in the amount of $67,872, net of issuance costs of approximately $128. In connection with this issuance, the Company used a portion of the proceeds received, approximately $18,000, 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 from existing stockholders. The difference between the amount implicitly paid to repurchase the various classes of Preferred Stock and the corresponding carrying value of the underlying shares, or $15,930, was treated as a deemed dividend and was recorded against retained earnings. As the shares of common stock were repurchased for constructive retirement, the excess purchase price over the corresponding par value was charged directly to retained earnings.

On August 23, 2016, the Company completed a Series E convertible preferred stock, or Series E Preferred Stock, offering 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 restricted stock units from existing stockholders. The difference between the amount implicitly paid to repurchase the various classes of Preferred Stock and the corresponding carrying value of the underlying shares, or $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, the excess purchase price over the corresponding par value was charged directly to retained earnings.

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 had authority to issue was (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 were designated Series A Preferred Stock, 3,329,497 shares were designated Series B Preferred Stock, 1,648,978 shares were designated Series C Preferred Stock, 1,673,105 shares were designated Series D Preferred Stock, and 1,107,202 shares were designated 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 fully paid and non‑assessable shares of outstanding Class A common stock and four fully paid and non‑assessable shares of outstanding Class B common stock, and (ii) each share of Class B common stock of the Company issued and outstanding was recapitalized, reclassified, and reconstituted into two fully paid and non‑assessable shares of outstanding Class A common stock and four 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 was exercisable for and each outstanding RSU was convertible into was adjusted such that upon exercise of outstanding stock options or vesting of outstanding RSUs, each holder would receive two fully paid and non‑assessable shares of Class A common stock and four 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 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 IPO, all of the outstanding shares of 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.

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 up to 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. As of December 31, 2017, the Preferred Stock is 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 (1) the death of Langley Steinert, the Company’s Chief Executive Officer, President and Chairman, (2) his voluntary termination of all employment with the Company and service on the Company’s board of directors, or (3) 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.

Common Stock

Each share of Class A common stock entitles the holder to one 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 December 31, 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 will occur upon the occurrence of a Transfer, as defined in the Fourth Amended and Restated Certificate of Incorporation, of such share of Class B common stock.

Upon the effectiveness of the Company’s Fourth Amended and Restated Certificate of Incorporation, additional terms of conversion and transfer were implemented as discussed above.

Preferred Stock

Prior to the Company’s IPO, at which time all shares of Preferred Stock were converted into shares of common stock, the Company’s Preferred Stock consisted of the following:

	Original Issue Price Per Share		Shares Authorized		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

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands			12 Months Ended
	Aug. 23, 2016	Jul. 07, 2015	Dec. 31, 2016	Dec. 31, 2015
Series D Preferred Stock
Stock issuance costs		$ 128		$ 130
Series E Preferred Stock
Stock issuance costs	$ 268		$ 280

Consolidated Statements of Cash Flows (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Statement Of Cash Flows [Abstract]
Initial public offering cost	$ 3,308