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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2018	Jul. 31, 2018
Document And Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
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		88,762,521
Class B Common Stock
Document And Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		20,702,084

Unaudited Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 31,762	$ 87,709
Investments	110,000	50,000
Accounts receivable, net of allowance for doubtful accounts of $551 and $494, respectively	11,432	12,577
Prepaid expenses, prepaid income taxes and other current assets	11,090	6,918
Total current assets	164,284	157,204
Property and equipment, net	16,221	16,563
Restricted cash	3,604	1,843
Deferred tax assets	29,049	825
Other long–term assets	143	159
Total assets	213,301	176,594
Current liabilities
Accounts payable	30,476	23,908
Accrued expenses, accrued income taxes and other current liabilities	11,290	13,588
Deferred revenue	7,577	4,305
Deferred rent	1,206	1,165
Total current liabilities	50,549	42,966
Deferred rent, net of current portion	5,206	5,648
Other non–current liabilities	1,155	955
Total liabilities	56,910	49,569
Commitments and contingencies (Note 6)
Stockholders’ equity:
Additional paid-in capital	179,716	185,190
Accumulated deficit	(23,583)	(58,499)
Accumulated other comprehensive income	148	228
Total stockholders’ equity	156,391	127,025
Total liabilities and stockholders’ equity	213,301	176,594
Class A Common Stock
Stockholders’ equity:
Common stock	89	78
Class B Common Stock
Stockholders’ equity:
Common stock	$ 21	$ 28

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

3 Months Ended

6 Months Ended

Jun. 30, 2018

Jun. 30, 2017

Jun. 30, 2018

Jun. 30, 2017

Income Statement [Abstract]

Revenue

$ 110,325

$ 76,240

$ 209,026

$ 143,275

Cost of revenue

^[1]

5,959

4,322

11,528

7,647

Gross profit

104,366

71,918

197,498

135,628

Operating expenses:

Sales and marketing

80,933

55,533

152,441

104,604

Product, technology, and development

11,844

4,709

20,942

8,357

General and administrative

9,541

5,033

17,412

9,092

Depreciation and amortization

604

648

1,337

1,196

Total operating expenses

102,922

65,923

192,132

123,249

Income from operations

1,444

5,995

5,366

12,379

Other income, net

703

985

217

Income before income taxes

2,147

6,048

6,351

12,596

(Benefit from) provision for income taxes

(29,118)

1,702

(28,565)

4,043

Net income

31,265

4,346

34,916

8,553

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

Net income

31,265

4,346

34,916

8,553

Net income attributable to participating securities

(2,563)

(5,045)

Net income attributable to common stockholders — basic

31,265

1,783

34,916

3,508

Net income attributable to participating securities

(2,468)

(4,853)

Net income attributable to common stockholders — diluted

$ 31,265

$ 1,878

$ 34,916

$ 3,700

Net income per share attributable to common stockholders: (Note 8)

Basic

$ 0.29

$ 0.04

$ 0.32

$ 0.08

Diluted

$ 0.28

$ 0.04

$ 0.31

$ 0.08

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

Basic

108,500,802

42,162,718

107,726,105

42,122,339

Diluted

113,081,209

46,097,163

113,215,564

46,182,359

[1]	Includes depreciation and amortization expense for the three months ended June 30, 2018 and 2017 and for the six months ended June 30, 2018 and 2017 of $616, $269, $1,120 and $391, respectively.

Unaudited Condensed Consolidated Statements of Stockholders' Equity - 6 months ended Jun. 30, 2018 - USD ($) $ in Thousands	Total	Class A Common Stock	Class B Common Stock	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ 127,025			$ 78	$ 28	$ 185,190	$ 228	$ (58,499)
Beginning balance, Shares at Dec. 31, 2017		77,884,754	28,226,104	77,884,754	28,226,104
Net income	34,916							34,916
Stock–based compensation expense	9,633					9,633
Issuance of common stock upon exercise of stock options	2,385			$ 3		2,382
Issuance of common stock upon exercise of stock options, Shares				2,473,318	10,690
Issuance of common stock upon vesting of restricted stock units				$ 1		(1)
Issuance of common stock upon vesting of restricted stock units, Shares				1,261,495
Payment of withholding taxes on net share settlements of equity awards	(17,488)					(17,488)
Payment of withholding taxes on net share settlements of equity awards, Shares				(471,470)
Conversion of common stock, value issued				$ 7
Conversion of common stock, shares issued				7,534,710
Conversion of common stock, shares converted					(7,534,710)
Conversion of common stock, value					$ (7)
Foreign currency translation adjustment	(80)						(80)
Ending balance at Jun. 30, 2018	$ 156,391			$ 89	$ 21	$ 179,716	$ 148	$ (23,583)
Ending balance, Shares at Jun. 30, 2018		88,682,807	20,702,084	88,682,807	20,702,084

Unaudited Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating Activities
Net income	$ 34,916	$ 8,553
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	2,457	1,587
Unrealized currency (gain) loss on foreign denominated transactions	(19)	128
Deferred taxes	(28,224)	410
Provision for doubtful accounts	722	380
Stock-based compensation expense	9,423	150
Changes in operating assets and liabilities:
Accounts receivable	418	(2,720)
Prepaid expenses, prepaid income taxes, and other assets	(4,312)	(890)
Accounts payable	7,338	1,200
Accrued expenses, accrued income taxes, and other current liabilities	(1,991)	(784)
Deferred revenue	3,315	1,251
Deferred rent	(434)	668
Other non-current liabilities	239	157
Net cash provided by operating activities	23,848	10,090
Investing Activities
Purchases of property and equipment	(981)	(1,976)
Capitalization of website development costs	(725)	(947)
Investments in certificates of deposit	(130,000)	(30,000)
Maturities of certificates of deposit	70,000	26,774
Net cash used in investing activities	(61,706)	(6,149)
Financing Activities
Proceeds from exercise of stock options	2,385	168
Payment of initial public offering costs	(1,142)	(305)
Payment of withholding taxes on net share settlements of equity awards	(17,488)
Net cash used in financing activities	(16,245)	(137)
Impact of foreign currency on cash, cash equivalents, and restricted cash	(83)	29
Net (decrease) increase in cash, cash equivalents, and restricted cash	(54,186)	3,833
Cash, cash equivalents, and restricted cash at beginning of period	89,552	31,520
Cash, cash equivalents, and restricted cash at end of period	35,366	35,353
Supplemental disclosure of cash flow information:
Cash paid for income taxes	2,280	647
Cash paid for interest	10	12
Supplemental disclosure of non-cash investing activities:
Unpaid purchases of property and equipment	712	2,271
Capitalized stockholders' compensation in website development costs	$ 210
Unpaid deferred initial public offering costs		$ 1,549

Summary of Significant Accounting Policies	6 Months Ended
Summary of Significant Accounting Policies	Jun. 30, 2018
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, 2017 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, filed with the Securities and Exchange Commission (“SEC”) on March 1, 2018 (the “2017 Annual Report”). 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 June 30, 2018 and December 31, 2017, results of operations for the three and six months ended June 30, 2018 and 2017, and cash flows for the six months ended June 30, 2018 and 2017. These interim period results 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 June 30, 2018, there have been no material changes in the Company's significant accounting policies from those that were disclosed in the 2017 Annual Report. 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. 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 Unaudited Condensed 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 Company’s initial public offering (“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. 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 ceases to be 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, at the end of its fiscal year when it had more than $700.0 million in market value of its stock held by non‑affiliates as of the last business day of the its most recently completed second fiscal quarter (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. Because the market value of the Company’s Class A common stock held by non-affiliates exceeded $700.0 million as of June 29, 2018, the Company will have been public for more than one year and it has filed its 2017 Annual Report, the Company will cease to be an emerging growth company as of December 31, 2018. As a result, beginning with the Company’s Annual Report on Form 10-K for the year ending December 31, 2018, the Company will be subject to certain requirements that apply to other public companies but did not previously apply to the Company due to its status as an emerging growth company, including the provisions of Section 404 of the Sarbanes-Oxley Act, which require that the Company’s independent registered public accounting firm provides an attestation report on the effectiveness of the Company’s internal control over financial reporting. 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 six months ended June 30, 2018 and the year ended December 31, 2017, no individual customer accounted for more than 10% of total revenue. As of June 30, 2018, three customers accounted for 20%, 14% and 14% of net accounts receivable, respectively. As of December 31, 2017, two customers accounted for 29% and 17% of net accounts receivable, respectively. No other individual customer accounted for more than 10% of net accounts receivable at June 30, 2018 or December 31, 2017. Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (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 Accounting Standards Update (“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. 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 certain public entities for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. For all other entities, including emerging growth companies, the guidance in ASU 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. Since the Company will cease to be an emerging growth company as of December 31, 2018, the Company is required to adopt the standard during 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 continuing to assess 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 the impact that this guidance will have on its financial statements and related disclosures 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; however, the Company has yet to determine if the impact will be material. 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. Other Recent Accounting Pronouncements In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718) (“ASU 2018-07”). ASU 2018-07 expands the scope of Topic 718, Compensation—Stock Compensation, to include share-based payment transactions for acquiring goods and services from non-employees. The amendments in this update state that an entity should apply the requirements of Topic 718 to non-employee awards except for specific guidance on inputs to an option pricing model and the attribution of cost (that is, the period of time over which share-based payment awards vest and the pattern of cost recognition over that period). The amendments specify that Topic 718 applies to all share-based payment transactions in which a grantor acquires goods or services to be used or consumed in a grantor’s own operations by issuing share-based payment awards. The amendments also clarify that Topic 718 does not apply to share-based payments used to effectively provide (1) financing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract accounted for under Topic 606, Revenue from Contracts with Customers. The amendments in this update are effective for public business entities for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. For all other entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted, but no earlier than an entity’s adoption date of Topic 606. The Company has assessed the impact of this guidance on its consolidated financial statements and does not deem it to be material. The Company plans to adopt the guidance on January 1, 2019. 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 certain 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, including emerging growth companies, the new standard is effective for annual periods beginning after December 15, 2019, with early adoption permitted. While the Company is still evaluating the impact this guidance may have on its consolidated financial statements, it anticipates that such guidance will materially impact its Balance Sheet.

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

6 Months Ended

Jun. 30, 2018

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 June 30, 2018 and at December 31, 2017:

	At June 30, 2018
	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:
Money market funds	$	1,085	$	—	$	—	$	1,085
Investments:
Certificates of deposit		—		110,000		—		110,000
Total	$	1,085	$	110,000	$	—	$	111,085

	At 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:
Money market funds	$	60,709	$	—	$	—	$	60,709
Investments:
Certificates of deposit		—		50,000		—		50,000
Total	$	60,709	$	50,000	$	—	$	110,709

Certificates of deposit at June 30, 2018 and December 31, 2017 had maturity dates of one year or less.

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 six months ended June 30, 2018 or the year ended December 31, 2017.

The Company considers all highly liquid investments with an original maturity of 90 days or less at the date of purchase to be cash equivalents. Investments not classified as cash equivalents with maturities one year or less 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 June 30, 2018 and December 31, 2017.


	At June 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Cash and cash equivalents due in 90 days or less	$	31,762	$	—	$	—	$	31,762
Investments:
Certificates of deposit due in one year or less		110,000						110,000
Total cash, cash equivalents, and investments	$	141,762	$	—	$	—	$	141,762


	At December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
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

Stock-based Compensation

6 Months Ended

Jun. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

7. Stock-based Compensation

For the three months ended June 30, 2018 and 2017, total stock-based compensation expense was $5,605 and $74, respectively. For the six months ended June 30, 2018 and 2017, total stock-based compensation expense was $9,423 and $150, respectively. The following two tables show stock compensation expense by award type and where the stock compensation expense is recorded in the Company’s Unaudited Condensed Consolidated Income Statements:

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Options	$	64	$	74	$	129	$	150
RSUs		5,541		—		9,294		—
Total stock-based compensation expense	$	5,605	$	74	$	9,423	$	150

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Cost of revenue	$	92	$	5	$	181	$	10
Sales and marketing expense		1,536		35		2,546		73
Product, technology, and development expense		2,658		23		4,319		48
General and administrative expense		1,319		11		2,377		19
Total	$	5,605	$	74	$	9,423	$	150

Excluded from stock-based compensation expense is $210 of capitalized website development costs in the six months ended June 30, 2018. Stock-based compensation expenses related to capitalized website development costs were immaterial for the three months ended June 30, 2018 and 2017 and for the six months ended June 30, 2017.

During the three months ended June 30, 2018, the Company withheld 471,470 shares of Class A common stock to satisfy employee tax withholding requirements due to net share settlement. The shares withheld remain in the authorized, but unissued pool under the Company’s Omnibus Equity Compensation Plan and can be reissued by the Company. Total payments of $17,488 for the employees’ tax obligations to the taxing authorities due to net share settlements are reflected as a financing activity within the Unaudited Condensed Consolidated Statements of Cash Flows.

Earnings Per Share

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

Earnings Per Share

8. Earnings Per Share

Net income per share for the three and six months ended June 30, 2018 is computed by dividing net income by the weighted-average number of common shares outstanding during the reporting period. The Company computes the weighted-average number of common shares outstanding during the reporting period using the total number of shares of Class A common stock and Class B common stock outstanding as of the last day of the previous year end reporting period plus the weighted-average of any additional shares issued and outstanding during the reporting period.

Net income per share for the three and six months ended June 30, 2017 is computed 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). As of June 30, 2017, the Company had convertible preferred stock outstanding. The Company considered the convertible preferred stock to be participating securities because they included rights to participate in dividends with the common stock. On October 16, 2017, in connection with 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. As a result, there were no shares of preferred stock outstanding at the closing of the IPO and the Company has not issued any new shares of preferred stock since such closing.

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 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 allocated net income first to preferred stockholders based on dividend rights under the Company’s certificate of incorporation that was in effect prior to the closing of the IPO and then to preferred and common stockholders based on ownership interests.

The Company has two classes of common stock authorized: Class A common stock and Class B common stock. The rights of the holders of Class A and Class B common stock are identical, except with respect to voting and conversion. Each share of Class A common stock is entitled to one vote per share and each share of Class B common stock is entitled to ten votes per share. Each share of Class B common stock is convertible into one share of Class A common stock at the option of the holder at any time or automatically upon certain events described in the Company’s amended and restated certificate of incorporation, including on either the death or voluntary termination of the Company’s Chief Executive Officer. The Company allocates undistributed earnings attributable to common stock between the common stock classes on a one‑to‑one basis when computing net income per share. As a result, basic and diluted net income per share of Class A common stock and per share of Class B common stock are equivalent.

During the six months ended June 30, 2018, holders of Class B common stock elected to convert 7,534,710 shares of Class B common stock to Class A common stock.

Diluted net income per share gives effect to all potentially dilutive securities. Potential dilutive securities for the three and six months ended June 30, 2018 and 2017 consist of shares of common stock issuable upon the exercise of stock options and shares of common stock issuable upon the vesting of RSUs. Potential dilutive securities for the three and six months ended June 30, 2017 also included shares of common stock issuable upon the conversion of the outstanding preferred stock. The dilutive effect of these common stock equivalents is reflected in diluted earnings per share by application of the treasury stock method.

For the three and six months ended June 30, 2018, dilutive net income per share is calculated by dividing net income by the weighted-average number of shares of common stock outstanding during the period plus the dilutive impact of stock options and shares of common stock issuable upon the vesting of RSUs. For the three and six months ended June 30, 2017, the two‑class method was used in the computation of diluted net income per share, which was equally as dilutive as the if-converted method.

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

	Three Months Ended June 30,					Six Months Ended June 30,
	2018		2017			2018		2017
Numerator:
Net income	$	31,265	$	4,346		$	34,916	$	8,553
Net income attributable to participating securities		—		(2,563	)		—		(5,045	)
Net income attributable to common stockholders — basic	$	31,265	$	1,783		$	34,916	$	3,508
Net income	$	31,265	$	4,346		$	34,916	$	8,553
Net income attributable to participating securities		—		(2,468	)		—		(4,853	)
Net income attributable to common stockholders — diluted	$	31,265	$	1,878		$	34,916	$	3,700
Denominator:
Weighted-average number of shares of common stock used in computing net income per share attributable to common stockholders — basic		108,500,802		42,162,718			107,726,105		42,122,339
Dilutive effect of share equivalents resulting from stock options		3,159,031		3,934,445			3,965,593		4,060,020
Dilutive effect of share equivalents resulting from unvested restricted stock units		1,421,376		—			1,523,866		—
Weighted-average number of shares of common stock used in computing net income per share — diluted		113,081,209		46,097,163			113,215,564		46,182,359
Net income per share attributable to common stockholders:
Basic	$	0.29	$	0.04		$	0.32	$	0.08
Diluted	$	0.28	$	0.04		$	0.31	$	0.08

The following potentially dilutive common stock equivalents have been excluded from the calculation of diluted weighted-average shares outstanding for the three and six months ended June 30, 2018 and 2017, as their effect would have been anti-dilutive for the period presented:

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Stock options outstanding		—		1,202,070		—		1,224,477
Restricted stock units outstanding		160,900		2,765,910		80,450		2,369,802
Convertible preferred stock		—		—		—		—

Segment and Geographic Information

6 Months Ended

Jun. 30, 2018

Segment Reporting [Abstract]

Segment and Geographic Information

10. 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 are 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 income (loss) 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 June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Segment revenue:
United States	$	106,397	$	74,142	$	201,564	$	139,560
International		3,928		2,098		7,462		3,715
Total revenue	$	110,325	$	76,240	$	209,026	$	143,275

	Three Months Ended June 30,						Six Months Ended June 30,
	2018			2017			2018			2017
Segment income (loss) from operations:
United States	$	9,659		$	12,152		$	21,244		$	24,280
International		(8,215	)		(6,157	)		(15,878	)		(11,901	)
Total income from operations	$	1,444		$	5,995		$	5,366		$	12,379

As of June 30, 2018 and December 31, 2017, property and equipment held outside the United States was not material.

Summary of Significant Accounting Policies (Policies)	6 Months Ended
Summary of Significant Accounting Policies (Policies)	Jun. 30, 2018
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, 2017 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, filed with the Securities and Exchange Commission (“SEC”) on March 1, 2018 (the “2017 Annual Report”). 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 June 30, 2018 and December 31, 2017, results of operations for the three and six months ended June 30, 2018 and 2017, and cash flows for the six months ended June 30, 2018 and 2017. These interim period results 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 June 30, 2018, there have been no material changes in the Company's significant accounting policies from those that were disclosed in the 2017 Annual Report.
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.
Use of Estimates	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 Unaudited Condensed 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 Company’s initial public offering (“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.
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 ceases to be 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, at the end of its fiscal year when it had more than $700.0 million in market value of its stock held by non‑affiliates as of the last business day of the its most recently completed second fiscal quarter (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. Because the market value of the Company’s Class A common stock held by non-affiliates exceeded $700.0 million as of June 29, 2018, the Company will have been public for more than one year and it has filed its 2017 Annual Report, the Company will cease to be an emerging growth company as of December 31, 2018. As a result, beginning with the Company’s Annual Report on Form 10-K for the year ending December 31, 2018, the Company will be subject to certain requirements that apply to other public companies but did not previously apply to the Company due to its status as an emerging growth company, including the provisions of Section 404 of the Sarbanes-Oxley Act, which require that the Company’s independent registered public accounting firm provides an attestation report on the effectiveness of the Company’s internal control over financial reporting.
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 six months ended June 30, 2018 and the year ended December 31, 2017, no individual customer accounted for more than 10% of total revenue. As of June 30, 2018, three customers accounted for 20%, 14% and 14% of net accounts receivable, respectively. As of December 31, 2017, two customers accounted for 29% and 17% of net accounts receivable, respectively. No other individual customer accounted for more than 10% of net accounts receivable at June 30, 2018 or December 31, 2017.
Recent Accounting Pronouncements	Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (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 Accounting Standards Update (“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. 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 certain public entities for annual reporting periods beginning after December 15, 2017, including interim periods within those annual reporting periods. For all other entities, including emerging growth companies, the guidance in ASU 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. Since the Company will cease to be an emerging growth company as of December 31, 2018, the Company is required to adopt the standard during 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 continuing to assess 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 the impact that this guidance will have on its financial statements and related disclosures 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; however, the Company has yet to determine if the impact will be material. 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. Other Recent Accounting Pronouncements In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718) (“ASU 2018-07”). ASU 2018-07 expands the scope of Topic 718, Compensation—Stock Compensation, to include share-based payment transactions for acquiring goods and services from non-employees. The amendments in this update state that an entity should apply the requirements of Topic 718 to non-employee awards except for specific guidance on inputs to an option pricing model and the attribution of cost (that is, the period of time over which share-based payment awards vest and the pattern of cost recognition over that period). The amendments specify that Topic 718 applies to all share-based payment transactions in which a grantor acquires goods or services to be used or consumed in a grantor’s own operations by issuing share-based payment awards. The amendments also clarify that Topic 718 does not apply to share-based payments used to effectively provide (1) financing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract accounted for under Topic 606, Revenue from Contracts with Customers. The amendments in this update are effective for public business entities for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. For all other entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted, but no earlier than an entity’s adoption date of Topic 606. The Company has assessed the impact of this guidance on its consolidated financial statements and does not deem it to be material. The Company plans to adopt the guidance on January 1, 2019. 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 certain 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, including emerging growth companies, the new standard is effective for annual periods beginning after December 15, 2019, with early adoption permitted. While the Company is still evaluating the impact this guidance may have on its consolidated financial statements, it anticipates that such guidance will materially impact its Balance Sheet.
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 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 six months ended June 30, 2018 or the year ended December 31, 2017. The Company considers all highly liquid investments with an original maturity of 90 days or less at the date of purchase to be cash equivalents. Investments not classified as cash equivalents with maturities one year or less 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.
Earnings Per Share	Net income per share for the three and six months ended June 30, 2018 is computed by dividing net income by the weighted-average number of common shares outstanding during the reporting period. The Company computes the weighted-average number of common shares outstanding during the reporting period using the total number of shares of Class A common stock and Class B common stock outstanding as of the last day of the previous year end reporting period plus the weighted-average of any additional shares issued and outstanding during the reporting period. Net income per share for the three and six months ended June 30, 2017 is computed 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). As of June 30, 2017, the Company had convertible preferred stock outstanding. The Company considered the convertible preferred stock to be participating securities because they included rights to participate in dividends with the common stock. On October 16, 2017, in connection with 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. As a result, there were no shares of preferred stock outstanding at the closing of the IPO and the Company has not issued any new shares of preferred stock since such closing. 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 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 allocated net income first to preferred stockholders based on dividend rights under the Company’s certificate of incorporation that was in effect prior to the closing of the IPO and then to preferred and common stockholders based on ownership interests. The Company has two classes of common stock authorized: Class A common stock and Class B common stock. The rights of the holders of Class A and Class B common stock are identical, except with respect to voting and conversion. Each share of Class A common stock is entitled to one vote per share and each share of Class B common stock is entitled to ten votes per share. Each share of Class B common stock is convertible into one share of Class A common stock at the option of the holder at any time or automatically upon certain events described in the Company’s amended and restated certificate of incorporation, including on either the death or voluntary termination of the Company’s Chief Executive Officer. The Company allocates undistributed earnings attributable to common stock between the common stock classes on a one‑to‑one basis when computing net income per share. As a result, basic and diluted net income per share of Class A common stock and per share of Class B common stock are equivalent.

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

6 Months Ended

Jun. 30, 2018

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 June 30, 2018 and at December 31, 2017:

	At June 30, 2018
	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:
Money market funds	$	1,085	$	—	$	—	$	1,085
Investments:
Certificates of deposit		—		110,000		—		110,000
Total	$	1,085	$	110,000	$	—	$	111,085

	At 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:
Money market funds	$	60,709	$	—	$	—	$	60,709
Investments:
Certificates of deposit		—		50,000		—		50,000
Total	$	60,709	$	50,000	$	—	$	110,709

Schedule of Cash, Cash Equivalents, and Investments

The following is a summary of cash, cash equivalents, and investments as of June 30, 2018 and December 31, 2017.


	At June 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Cash and cash equivalents due in 90 days or less	$	31,762	$	—	$	—	$	31,762
Investments:
Certificates of deposit due in one year or less		110,000						110,000
Total cash, cash equivalents, and investments	$	141,762	$	—	$	—	$	141,762


	At December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
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

Stock-based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock-based Compensation Expense by Award Type

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Options	$	64	$	74	$	129	$	150
RSUs		5,541		—		9,294		—
Total stock-based compensation expense	$	5,605	$	74	$	9,423	$	150

Summary of Allocation of Stock-based Compensation Expense

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Cost of revenue	$	92	$	5	$	181	$	10
Sales and marketing expense		1,536		35		2,546		73
Product, technology, and development expense		2,658		23		4,319		48
General and administrative expense		1,319		11		2,377		19
Total	$	5,605	$	74	$	9,423	$	150

Earnings Per Share (Tables)

6 Months Ended

Jun. 30, 2018

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 per share:

	Three Months Ended June 30,					Six Months Ended June 30,
	2018		2017			2018		2017
Numerator:
Net income	$	31,265	$	4,346		$	34,916	$	8,553
Net income attributable to participating securities		—		(2,563	)		—		(5,045	)
Net income attributable to common stockholders — basic	$	31,265	$	1,783		$	34,916	$	3,508
Net income	$	31,265	$	4,346		$	34,916	$	8,553
Net income attributable to participating securities		—		(2,468	)		—		(4,853	)
Net income attributable to common stockholders — diluted	$	31,265	$	1,878		$	34,916	$	3,700
Denominator:
Weighted-average number of shares of common stock used in computing net income per share attributable to common stockholders — basic		108,500,802		42,162,718			107,726,105		42,122,339
Dilutive effect of share equivalents resulting from stock options		3,159,031		3,934,445			3,965,593		4,060,020
Dilutive effect of share equivalents resulting from unvested restricted stock units		1,421,376		—			1,523,866		—
Weighted-average number of shares of common stock used in computing net income per share — diluted		113,081,209		46,097,163			113,215,564		46,182,359
Net income per share attributable to common stockholders:
Basic	$	0.29	$	0.04		$	0.32	$	0.08
Diluted	$	0.28	$	0.04		$	0.31	$	0.08

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Stock options outstanding		—		1,202,070		—		1,224,477
Restricted stock units outstanding		160,900		2,765,910		80,450		2,369,802
Convertible preferred stock		—		—		—		—

Segment and Geographic Information (Tables)

6 Months Ended

Jun. 30, 2018

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 June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Segment revenue:
United States	$	106,397	$	74,142	$	201,564	$	139,560
International		3,928		2,098		7,462		3,715
Total revenue	$	110,325	$	76,240	$	209,026	$	143,275

	Three Months Ended June 30,						Six Months Ended June 30,
	2018			2017			2018			2017
Segment income (loss) from operations:
United States	$	9,659		$	12,152		$	21,244		$	24,280
International		(8,215	)		(6,157	)		(15,878	)		(11,901	)
Total income from operations	$	1,444		$	5,995		$	5,366		$	12,379

Summary of Significant Accounting Policies - Additional Information (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) Customer	Dec. 31, 2017 Customer	Jun. 29, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
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, at the end of its fiscal year when it had more than $700.0 million in market value of its stock held by nonaffiliates as of the last business day of the its most recently completed second fiscal quarter (and it has been a public company for at least 12 months, and has filed one Annual Report on Form 10K), or it issues more than $1.0 billion of nonconvertible debt securities over a threeyear period.
Debt security maturity period	3 years
Description of significant off-balance sheet risk	The Company has no significant offbalance 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%
Number of major customers \| Customer	0	0
Net Accounts Receivable \| Concentration of Credit Risk
Summary Of Significant Accounting Policies [Line Items]
Number of major customers \| Customer	0	0
Net Accounts Receivable \| Concentration of Credit Risk \| Customer One
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk, percentage	20.00%	29.00%
Net Accounts Receivable \| Concentration of Credit Risk \| Customer Two
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk, percentage	14.00%	17.00%
Net Accounts Receivable \| Concentration of Credit Risk \| Customer Three
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk, percentage	14.00%
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
Non convertible debt securities	$ 1,000,000,000
Minimum \| Class A Common Stock
Summary Of Significant Accounting Policies [Line Items]
Market value of common stock held by non-affiliates			$ 700,000,000

Earnings Per Share - Additional Information (Details)		6 Months Ended
Earnings Per Share - Additional Information (Details)	Oct. 16, 2017 shares	Jun. 30, 2018 Vote shares
Earnings Per Share Basic [Line Items]
Convertible preferred stock, terms of conversion		On October 16, 2017, in connection with 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. As a result, there were no shares of preferred stock outstanding at the closing of the IPO and the Company has not issued any new shares of preferred stock since such closing
Conversion of stock, description		Each share of Class B common stock is convertible into one share of Class A common stock at the option of the holder at any time or automatically upon certain events described in the Company’s amended and restated certificate of incorporation, including on either the death or voluntary termination of the Company’s Chief Executive Officer.
Undistributed earnings ratio used to calculate allocation to class of stock		100.00%
IPO
Earnings Per Share Basic [Line Items]
Preferred stock, shares outstanding	0
Class A Common Stock
Earnings Per Share Basic [Line Items]
Right to voting		one vote per share
Number of votes entitled to stockholders per share \| Vote		1
Conversion of stock		7,534,710
Class A Common Stock \| IPO
Earnings Per Share Basic [Line Items]
Conversion of preferred stock	20,188,226
Class B Common Stock
Earnings Per Share Basic [Line Items]
Conversion of preferred stock	40,376,452
Right to voting		ten votes per 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		1

Earnings Per Share - Schedule of Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income	$ 31,265	$ 4,346	$ 34,916	$ 8,553
Net income attributable to participating securities		(2,563)		(5,045)
Net income attributable to common stockholders — basic	31,265	1,783	34,916	3,508
Net income attributable to participating securities		(2,468)		(4,853)
Net income attributable to common stockholders — diluted	$ 31,265	$ 1,878	$ 34,916	$ 3,700
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	108,500,802	42,162,718	107,726,105	42,122,339
Dilutive effect of share equivalents resulting from stock options	3,159,031	3,934,445	3,965,593	4,060,020
Dilutive effect of share equivalents resulting from unvested restricted stock units	1,421,376		1,523,866
Weighted-average number of shares of common stock used in computing net income per share — diluted	113,081,209	46,097,163	113,215,564	46,182,359
Net income per share attributable to common stockholders:
Basic	$ 0.29	$ 0.04	$ 0.32	$ 0.08
Diluted	$ 0.28	$ 0.04	$ 0.31	$ 0.08

Unaudited Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Allowance for doubtful accounts	$ 551	$ 494
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	88,682,807	77,884,754
Common stock, shares outstanding	88,682,807	77,884,754
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	20,702,084	28,226,104
Common stock, shares outstanding	20,702,084	28,226,104

Organization and Business Description	6 Months Ended
Organization and Business Description	Jun. 30, 2018
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, Germany, Italy, and Spain. The Company has wholly owned subsidiaries in the United States, Canada, Ireland, and the United Kingdom. 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.

Commitments and Contingencies	6 Months Ended
Commitments and Contingencies	Jun. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Operating Leases The Company’s lease obligations consist of various leases for office space in: Cambridge, Massachusetts; Detroit, Michigan; and Dublin, Ireland, with various lease terms through 2033. 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 June 30, 2018 and all of the Company’s property, equipment, and software have been purchased with cash with the exception of $712 of unpaid property and equipment costs as of June 30, 2018. The Company has no material long-term purchase obligations outstanding with any vendors or third parties. As of June 30, 2018, there were no material changes in the Company’s contractual obligations and commitments from those disclosed in the 2017 Annual Report, other than as discussed below. On June 19, 2018, the Company entered into an additional operating lease in Cambridge, Massachusetts, comprised of 48,393 square feet of office space and with a non-cancellable lease term through 2033. Under the terms of the lease, the Company will pay an annual fixed rent of $68.00 per rentable square foot, resulting in an aggregate annual fixed rent of $3,291, subject to an annual increase through the term of the lease. At June 30, 2018 and December 31, 2017, restricted cash was $3,604 and $1,843, respectively, and primarily related to cash held at a financial institution in an interest‑bearing cash account as collateral for three letters of credit related to the contractual provisions for the Company’s building lease security deposits. As of June 30, 2018 and December 31, 2017, the restricted cash is classified as a long‑term asset. 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 June 30, 2018 and December 31, 2017, the Company has not incurred any costs for guarantees or indemnities.

Income Taxes	6 Months Ended
Income Taxes	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The Tax Cuts and Jobs Act (the “TCJA”) was enacted on December 22, 2017. Among other things, the TCJA reduces the U.S. federal corporate tax rate from 35% to 21%, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred, and creates new taxes on certain foreign sourced earnings. The Company is still examining certain aspects of the TCJA and refining its calculations, which could potentially affect the measurement of these balances or potentially give rise to new deferred tax amounts. The Company recorded a provisional amount of $187 of tax expense during the year ended December 31, 2017. The Company recorded an additional $205 of tax expense during the six months ended June 30, 2018 pertaining to a change in the estimated impact of the re-measurement of the Company’s U.S. net deferred tax assets. During the six months ended June 30, 2018, the Company recorded an income tax benefit of $28,565, representing an effective tax rate of (449.8)%. The effective tax rate for the six months ended June 30, 2018 was lower than the statutory tax rate of 21% principally due to the excess stock deductions from the taxable compensation of stock-based awards and estimated federal and state research and development tax credits, partially offset by state and local income taxes. During the six months ended June 30, 2017, the Company recorded income tax expense of $4,043, representing an effective tax rate of 32.1%. The effective tax rate for the six months ended June 30, 2017 was lower than the statutory tax rate of 35% principally due to federal and state research and development tax credits and excess stock deductions from the taxable compensation of stock-based awards, partially offset by state and local income taxes.

Organization and Business Description - Additional Information (Details)	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
State of incorporation	Delaware
Date of incorporation	Jun. 26, 2015

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) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	$ 31,762	$ 87,709
Investments:	110,000	50,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments:	$ 110,000	$ 50,000
Debt Securities, Held-to-maturity, Type [Extensible List]	us-gaap:CertificatesOfDepositMember	us-gaap:CertificatesOfDepositMember
Total	$ 111,085	$ 110,709
Fair Value, Measurements, Recurring \| 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	1,085	60,709
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2 Inputs)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments:	$ 110,000	$ 50,000
Debt Securities, Held-to-maturity, Type [Extensible List]	us-gaap:CertificatesOfDepositMember	us-gaap:CertificatesOfDepositMember
Total	$ 110,000	$ 50,000
Fair Value, Measurements, Recurring \| Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents:	1,085	60,709
Fair Value, Measurements, Recurring \| 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:	$ 1,085	$ 60,709

Property and Equipment, Net - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 25,307	$ 23,865
Less accumulated depreciation	(9,086)	(7,302)
Property and equipment, net	16,221	16,563
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,859	3,532
Website and Software Development Costs
Property Plant And Equipment [Line Items]
Property and equipment, gross	220	174
Website Development Costs
Property Plant And Equipment [Line Items]
Property and equipment, gross	5,813	4,895
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,542	4,421
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	10,805	10,797
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 68	$ 46

Accrued Expenses, Accrued Income Taxes and Other Current Liabilities - Schedule of Accrued Expenses, Accrued Income Taxes and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Accrued bonuses	$ 4,740	$ 7,807
Accrued commissions	2,603	1,581
Other accrued expenses, accrued income taxes and other current liabilities	3,947	4,200
Accrued expenses, accrued income taxes and other current liabilities, Total	$ 11,290	$ 13,588

Commitments and Contingencies - Additional Information (Details)		6 Months Ended		12 Months Ended
	Jun. 19, 2018 USD ($) ft²	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Commitments And Contingencies [Line Items]
Unpaid purchases of property and equipment		$ 712,000	$ 2,271,000
Restricted cash		3,604,000		$ 1,843,000
Cost for guarantees and indemnities		$ 0		$ 0
Cambridge, Massachusetts
Commitments And Contingencies [Line Items]
Square feet of rentable office space \| ft²	48,393
Annual fixed rent payable per square foot	$ 68.00
Operating lease, aggregate annual fixed rent	$ 3,291,000

Segment and Geographic Information - Additional Information (Details)	6 Months Ended
	Jun. 30, 2018 Segment
Segment Reporting [Abstract]
Number of reportable segments	2