Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	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