Condensed Consolidating Financial Information - Condensed Consolidating Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Income Statements, Captions [Line Items]
REVENUES	$ 2,943,462	$ 2,672,195	$ 5,803,488	$ 5,191,024
COST OF SALES	2,505,299	2,267,303	4,945,562	4,402,610
GROSS PROFIT	438,163	404,892	857,926	788,414
SELLING, GENERAL AND ADMINISTRATIVE EXPENSES	308,092	298,568	632,439	588,347
DEPRECIATION AND AMORTIZATION EXPENSE	16,638	14,093	32,980	27,699
ASSET IMPAIRMENTS	4,268	0	4,268	0
INCOME FROM OPERATIONS	109,165	92,231	188,239	172,368
Floorplan interest expense	(14,563)	(13,226)	(28,650)	(25,168)
Other interest expense, net	(19,414)	(17,315)	(38,234)	(34,314)
INCOME BEFORE INCOME TAXES	75,188	61,690	121,355	112,886
PROVISION FOR INCOME TAXES	(18,725)	(22,557)	(29,078)	(39,814)
EQUITY IN EARNINGS OF SUBSIDIARIES	0	0	0	0
NET INCOME	56,463	39,133	92,277	73,072
COMPREHENSIVE LOSS	(20,408)	3,881	(3,132)	10,676
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT	36,055	43,014	89,145	83,748
Elimination
Condensed Income Statements, Captions [Line Items]
REVENUES	0	0	0	0
COST OF SALES	0	0	0	0
GROSS PROFIT	0	0	0	0
SELLING, GENERAL AND ADMINISTRATIVE EXPENSES	0	0	0	0
DEPRECIATION AND AMORTIZATION EXPENSE	0	0	0	0
ASSET IMPAIRMENTS	0		0
INCOME FROM OPERATIONS	0	0	0	0
Floorplan interest expense	0	0	0	0
Other interest expense, net	0	0	0	0
INCOME BEFORE INCOME TAXES	0	0	0	0
PROVISION FOR INCOME TAXES	0	0	0	0
EQUITY IN EARNINGS OF SUBSIDIARIES	(56,945)	(39,467)	(94,103)	(74,634)
NET INCOME	(56,945)	(39,467)	(94,103)	(74,634)
COMPREHENSIVE LOSS	0	0	0	0
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT	(56,945)	(39,467)	(94,103)	(74,634)
Group 1 Automotive, Inc.
Condensed Income Statements, Captions [Line Items]
REVENUES	0	0	0	0
COST OF SALES	0	0	0	0
GROSS PROFIT	0	0	0	0
SELLING, GENERAL AND ADMINISTRATIVE EXPENSES	635	533	2,403	2,499
DEPRECIATION AND AMORTIZATION EXPENSE	0	0	0	0
ASSET IMPAIRMENTS	0		0
INCOME FROM OPERATIONS	(635)	(533)	(2,403)	(2,499)
Floorplan interest expense	0	0	0	0
Other interest expense, net	0	0	0	0
INCOME BEFORE INCOME TAXES	(635)	(533)	(2,403)	(2,499)
PROVISION FOR INCOME TAXES	153	200	577	937
EQUITY IN EARNINGS OF SUBSIDIARIES	56,945	39,467	94,103	74,634
NET INCOME	56,463	39,134	92,277	73,072
COMPREHENSIVE LOSS	0	0	0	0
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT	56,463	39,134	92,277	73,072
Guarantor Subsidiaries
Condensed Income Statements, Captions [Line Items]
REVENUES	2,168,001	2,123,691	4,256,488	4,091,409
COST OF SALES	1,817,331	1,783,218	3,570,133	3,430,341
GROSS PROFIT	350,670	340,473	686,355	661,068
SELLING, GENERAL AND ADMINISTRATIVE EXPENSES	231,923	242,014	481,088	474,625
DEPRECIATION AND AMORTIZATION EXPENSE	13,040	11,926	25,921	23,493
ASSET IMPAIRMENTS	4,268		4,268
INCOME FROM OPERATIONS	101,439	86,533	175,078	162,950
Floorplan interest expense	(12,810)	(12,062)	(25,147)	(22,940)
Other interest expense, net	(17,331)	(16,568)	(34,348)	(32,842)
INCOME BEFORE INCOME TAXES	71,298	57,903	115,583	107,168
PROVISION FOR INCOME TAXES	(17,556)	(21,895)	(27,336)	(38,979)
EQUITY IN EARNINGS OF SUBSIDIARIES	0	0	0	0
NET INCOME	53,742	36,008	88,247	68,189
COMPREHENSIVE LOSS	3,778	(581)	13,183	2,076
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT	57,520	35,427	101,430	70,265
Non-Guarantor Subsidiaries
Condensed Income Statements, Captions [Line Items]
REVENUES	775,461	548,504	1,547,000	1,099,615
COST OF SALES	687,968	484,085	1,375,429	972,269
GROSS PROFIT	87,493	64,419	171,571	127,346
SELLING, GENERAL AND ADMINISTRATIVE EXPENSES	75,534	56,021	148,948	111,223
DEPRECIATION AND AMORTIZATION EXPENSE	3,598	2,167	7,059	4,206
ASSET IMPAIRMENTS	0		0
INCOME FROM OPERATIONS	8,361	6,231	15,564	11,917
Floorplan interest expense	(1,753)	(1,164)	(3,503)	(2,228)
Other interest expense, net	(2,083)	(747)	(3,886)	(1,472)
INCOME BEFORE INCOME TAXES	4,525	4,320	8,175	8,217
PROVISION FOR INCOME TAXES	(1,322)	(862)	(2,319)	(1,772)
EQUITY IN EARNINGS OF SUBSIDIARIES	0	0	0	0
NET INCOME	3,203	3,458	5,856	6,445
COMPREHENSIVE LOSS	(24,186)	4,462	(16,315)	8,600
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT	$ (20,983)	$ 7,920	$ (10,459)	$ 15,045