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Cover - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2020
Feb. 12, 2021
Jun. 30, 2020
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2020    
Document Transition Report false    
Entity File Number 1-13461    
Entity Registrant Name Group 1 Automotive, Inc    
Entity Central Index Key 0001031203    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 76-0506313    
Entity Address, Address Line One 800 Gessner,    
Entity Address, Address Line Two Suite 500    
Entity Address, Postal Zip Code 77024    
Entity Address, City or Town Houston,    
Entity Address, State or Province TX    
City Area Code 713    
Local Phone Number 647-5700    
Entity Interactive Data Current Yes    
Title of 12(b) Security Common stock, par value $0.01 per share    
Trading Symbol GPI    
Security Exchange Name NYSE    
Amendment Flag false    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Entity Emerging Growth Company false    
Entity Small Business false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 1.1
Entity Common Stock, Shares Outstanding   18,095,702  
Documents Incorporated by Reference DOCUMENTS INCORPORATED BY REFERENCEPortions of the registrant’s definitive proxy statement for its 2021 Annual Meeting of Stockholders, which will be filed with the Securities and Exchange Commission within 120 days of December 31, 2020, are incorporated by reference into Part III of this Form 10-K.