Condensed Consolidating Financial Information - Condensed Consolidating Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Condensed Income Statements, Captions [Line Items]
REVENUES	$ 2,889,058	$ 3,012,292	$ 8,692,546	$ 8,203,316
COST OF SALES	2,453,957	2,580,872	7,399,519	6,983,482
GROSS PROFIT	435,101	431,420	1,293,027	1,219,834
SELLING, GENERAL AND ADMINISTRATIVE EXPENSES	316,771	328,327	949,210	916,674
DEPRECIATION AND AMORTIZATION EXPENSE	16,981	15,059	49,961	42,758
ASSET IMPAIRMENTS	23,159	9,526	27,427	9,526
INCOME FROM OPERATIONS	78,190	78,508	266,429	250,876
Floorplan interest expense	(14,685)	(13,491)	(43,335)	(38,659)
Other interest expense, net	(19,140)	(17,874)	(57,374)	(52,188)
INCOME BEFORE INCOME TAXES	44,365	47,143	165,720	160,029
PROVISION FOR INCOME TAXES	(9,587)	(17,262)	(38,666)	(57,076)
EQUITY IN EARNINGS OF SUBSIDIARIES	0	0	0	0
NET INCOME	34,778	29,881	127,054	102,953
COMPREHENSIVE LOSS	(2,386)	9,853	(5,518)	20,529
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT	32,392	39,734	121,536	123,482
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	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	(35,042)	(30,151)	(129,145)	(104,785)
NET INCOME	(35,042)	(30,151)	(129,145)	(104,785)
COMPREHENSIVE LOSS	0	0	0	0
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT	(35,042)	(30,151)	(129,145)	(104,785)
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	323	433	2,726	2,932
DEPRECIATION AND AMORTIZATION EXPENSE	0	0	0	0
ASSET IMPAIRMENTS	0	0	0	0
INCOME FROM OPERATIONS	(323)	(433)	(2,726)	(2,932)
Floorplan interest expense	0	0	0	0
Other interest expense, net	0	0	0	0
INCOME BEFORE INCOME TAXES	(323)	(433)	(2,726)	(2,932)
PROVISION FOR INCOME TAXES	58	163	635	1,100
EQUITY IN EARNINGS OF SUBSIDIARIES	35,042	30,151	129,145	104,785
NET INCOME	34,777	29,881	127,054	102,953
COMPREHENSIVE LOSS	0	0	0	0
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT	34,777	29,881	127,054	102,953
Guarantor Subsidiaries
Condensed Income Statements, Captions [Line Items]
REVENUES	2,200,230	2,301,958	6,456,718	6,393,367
COST OF SALES	1,847,757	1,948,390	5,417,890	5,378,731
GROSS PROFIT	352,473	353,568	1,038,828	1,014,636
SELLING, GENERAL AND ADMINISTRATIVE EXPENSES	240,167	259,119	721,255	733,744
DEPRECIATION AND AMORTIZATION EXPENSE	13,510	12,380	39,431	35,873
ASSET IMPAIRMENTS	23,159	9,526	27,427	9,526
INCOME FROM OPERATIONS	75,637	72,543	250,715	235,493
Floorplan interest expense	(12,903)	(12,014)	(38,050)	(34,954)
Other interest expense, net	(17,272)	(16,726)	(51,620)	(49,568)
INCOME BEFORE INCOME TAXES	45,462	43,803	161,045	150,971
PROVISION FOR INCOME TAXES	(9,120)	(16,423)	(36,457)	(55,402)
EQUITY IN EARNINGS OF SUBSIDIARIES	0	0	0	0
NET INCOME	36,342	27,380	124,588	95,569
COMPREHENSIVE LOSS	3,522	1,454	16,705	3,531
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT	39,864	28,834	141,293	99,100
Non-Guarantor Subsidiaries
Condensed Income Statements, Captions [Line Items]
REVENUES	688,828	710,334	2,235,828	1,809,949
COST OF SALES	606,200	632,482	1,981,629	1,604,751
GROSS PROFIT	82,628	77,852	254,199	205,198
SELLING, GENERAL AND ADMINISTRATIVE EXPENSES	76,281	68,775	225,229	179,998
DEPRECIATION AND AMORTIZATION EXPENSE	3,471	2,679	10,530	6,885
ASSET IMPAIRMENTS	0	0	0	0
INCOME FROM OPERATIONS	2,876	6,398	18,440	18,315
Floorplan interest expense	(1,782)	(1,477)	(5,285)	(3,705)
Other interest expense, net	(1,868)	(1,148)	(5,754)	(2,620)
INCOME BEFORE INCOME TAXES	(774)	3,773	7,401	11,990
PROVISION FOR INCOME TAXES	(525)	(1,002)	(2,844)	(2,774)
EQUITY IN EARNINGS OF SUBSIDIARIES	0	0	0	0
NET INCOME	(1,299)	2,771	4,557	9,216
COMPREHENSIVE LOSS	(5,908)	8,399	(22,223)	16,998
COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO PARENT	$ (7,207)	$ 11,170	$ (17,666)	$ 26,214