v3.8.0.1
Document and Entity Information - shares
3 Months Ended
Mar. 31, 2018
Apr. 30, 2018
Document And Entity Information [Line Items]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Mar. 31, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q1  
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   87,234,652
Class B Common Stock    
Document And Entity Information [Line Items]    
Entity Common Stock, Shares Outstanding   20,702,084