Document and Entity Information - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Feb. 12, 2018	Jun. 30, 2017	Dec. 31, 2016
Document and Entity Information [Abstract]
Entity Registrant Name	GROUP 1 AUTOMOTIVE INC
Entity Central Index Key	0001031203
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Document Fiscal Year Focus	2017
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 Public Float			$ 1,226.2
Entity Common Stock, Shares Outstanding		20,911,543
Common Stock, Par or Stated Value Per Share	$ 0.01			$ 0.01