| Document and Entity Information - shares | 6 Months Ended | |
| Jun. 30, 2019 | Jul. 31, 2019 | |
| Document And Entity Information [Line Items] | ||
| Document Type | 10-Q/A | |
| Amendment Flag | true | |
| Amendment Description | This Form 10-Q/A is filed as an amendment (“Amendment No. 1”) to the Quarterly Report on Form 10-Q for the quarter ended June 30, 2019, filed on August 6, 2019 (the “Original Form 10-Q”) by CarGurus, Inc. (the “Company”), solely to correct a clerical error on the cover page of the Original Form 10-Q, which incorrectly stated that the number of shares of the Company’s Class A common stock, $0.001 par value per share (the “Class A Common Stock”), outstanding as of July 31, 2019 was 111,581,300. The cover page of this Amendment No. 1 correctly states that the number of shares of the Company’s Class A Common Stock outstanding on July 31, 2019 was 90,879,216. No other changes have been made to the Original Form 10-Q. This Amendment No. 1 is presented as of the filing date of the Original Form 10-Q and does not reflect events occurring after that date, or modify or update disclosures in any way other than as required to reflect the correction previously described. | |
| Document Period End Date | Jun. 30, 2019 | |
| Document Fiscal Year Focus | 2019 | |
| 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 | Large Accelerated Filer | |
| Entity Small Business | false | |
| Entity Emerging Growth Company | false | |
| Entity Shell Company | false | |
| Entity Current Reporting Status | Yes | |
| Entity File Number | 001-38233 | |
| Entity Tax Identification Number | 043843478 | |
| Entity Address, Address Line One | 2 Canal Park | |
| Entity Address, Address Line Two | 4th Floor | |
| Entity Address, City or Town | Cambridge | |
| Entity Address, State or Province | Massachusetts | |
| Entity Address, Postal Zip Code | 02141 | |
| City Area Code | 617 | |
| Local Phone Number | 354-0068 | |
| Class A Common Stock | ||
| Document And Entity Information [Line Items] | ||
| Entity Common Stock, Shares Outstanding | 90,879,216 | |
| Class B Common Stock | ||
| Document And Entity Information [Line Items] | ||
| Entity Common Stock, Shares Outstanding | 20,702,084 |
