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 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 September 30, 2018 and December 31, 2017, results of operations and comprehensive income for the three and nine months ended September 30, 2018 and 2017, changes in stockholders’ equity as of September 30, 2018, and cash flows for the nine months ended September 30, 2018 and 2017. In the results of operations for the three and nine months ended September 30, 2018 and 2017, the Company has separately presented interest income from other income, net due to the increase in amount during the three and nine months ended September 30, 2018 as compared to the three and nine months ended September 30, 2017. In the Company’s financial position as of September 30, 2018 and December 31, 2017, the Company has separately presented other current assets from prepaid expenses and prepaid income taxes due to the increase in the balance at September 30, 2018 as compared to December 31, 2017. 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 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 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 as of December 31, 2018 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 provide 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 nine months ended September 30, 2018 and the year ended December 31, 2017, no individual customer accounted for more than 10% of total revenue. As of September 30, 2018 , three customers accounted for 19%, 16% and 13% 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 September 30, 2018 or December 31, 2017. Included in net accounts receivable at September 30, 2018 and December 31, 2017, is $4,428 and $1,845 of unbilled accounts receivables related to advertising customers billed within a quarter subsequent to services rendered. Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (the “FASB”) Revenue Recognition In May 2014, the FASB issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers 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 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 and prior to the filing of the Annual Report on Form 10-K for the year ended December 31, 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. Lease Accounting In February 2016, the FASB issued ASU No. 2016-02 , Leases (Topic 842) In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements Other Recent Accounting Pronouncements In June 2018, the FASB issued ASU No. 2018-07 , Compensation—Stock Compensation (Topic 718) Compensation—Stock Compensation |