Condensed Consolidated Financial Information Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	$ 28,787	$ 20,992	$ 13,037	$ 40,975
Contracts-in-transit and vehicle receivables, net	306,433	269,508
Accounts and notes receivable, net	188,611	173,364
Intercompany accounts receivable	0	0
Inventory, Net	1,763,293	1,651,815
Prepaid expenses and other current assets	42,062	34,908
Total current assets	2,329,186	2,150,587
PROPERTY AND EQUIPMENT, net	1,318,959	1,125,883
GOODWILL	913,034	876,763	854,915
INTANGIBLE FRANCHISE RIGHTS	285,632	284,876
INVESTMENT IN SUBSIDIARIES	0	0
OTHER ASSETS	24,254	23,794
Total assets	4,871,065	4,461,903
Floorplan notes payable - credit facility and other	1,240,695	1,136,654
Offset account related to floorplan notes payable - credit facility	(86,547)	(59,626)
Floorplan notes payable - manufacturer affiliates	397,183	392,661
FMCC Offset	(22,500)	(25,500)
Current maturities of long-term debt and short-term financing	77,609	72,419
Derivative Liability, Current	1,996	3,941
Accounts payable	412,981	356,099
Intercompany accounts payable	0	0
Accrued expenses	177,070	176,469
Total current liabilities	2,198,487	2,053,117
LONG-TERM DEBT, net of current maturities	1,318,184	1,212,809
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	8,583	20,470
DEFERRED INCOME TAXES AND OTHER LIABILITIES	221,529	245,307
Group 1 stockholders’ equity	1,124,282	930,200	918,252	978,010
Intercompany note receivable	0	0
Total stockholders’ equity	1,124,282	930,200
Total liabilities and stockholders' equity	4,871,065	4,461,903
Consolidation, Eliminations [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	0	0
Contracts-in-transit and vehicle receivables, net	0	0
Accounts and notes receivable, net	0	0
Intercompany accounts receivable	(39,936)	(8,929)
Inventory, Net	0	0
Prepaid expenses and other current assets	0	0
Total current assets	(39,936)	(8,929)
PROPERTY AND EQUIPMENT, net	0	0
GOODWILL	0	0
INTANGIBLE FRANCHISE RIGHTS	0	0
INVESTMENT IN SUBSIDIARIES	(2,999,407)	(2,787,328)
OTHER ASSETS	0	0
Total assets	(3,039,343)	(2,796,257)
Floorplan notes payable - credit facility and other	0	0
Offset account related to floorplan notes payable - credit facility	0	0
Floorplan notes payable - manufacturer affiliates	0	0
FMCC Offset	0	0
Current maturities of long-term debt and short-term financing	0	0
Derivative Liability, Current	0	0
Accounts payable	0	0
Intercompany accounts payable	(930,931)	(884,591)
Accrued expenses	0	0
Total current liabilities	(930,931)	(884,591)
LONG-TERM DEBT, net of current maturities	0	0
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0	0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	0	0
Group 1 stockholders’ equity	(2,999,407)	(2,787,328)
Intercompany note receivable	890,995	875,662
Total stockholders’ equity	(2,108,412)	(1,911,666)
Total liabilities and stockholders' equity	(3,039,343)	(2,796,257)
Parent Company [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	0	0	0	0
Contracts-in-transit and vehicle receivables, net	0	0
Accounts and notes receivable, net	0	0
Intercompany accounts receivable	26,988	0
Inventory, Net	0	0
Prepaid expenses and other current assets	1,934	516
Total current assets	28,922	516
PROPERTY AND EQUIPMENT, net	0	0
GOODWILL	0	0
INTANGIBLE FRANCHISE RIGHTS	0	0
INVESTMENT IN SUBSIDIARIES	2,999,407	2,787,328
OTHER ASSETS	0	0
Total assets	3,028,329	2,787,844
Floorplan notes payable - credit facility and other	0	0
Offset account related to floorplan notes payable - credit facility	0	0
Floorplan notes payable - manufacturer affiliates	0	0
FMCC Offset	0	0
Current maturities of long-term debt and short-term financing	24,741	0
Derivative Liability, Current	0	0
Accounts payable	0	0
Intercompany accounts payable	890,995	875,662
Accrued expenses	0	0
Total current liabilities	915,736	875,662
LONG-TERM DEBT, net of current maturities	865,202	836,056
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0	0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	(117)	(1,020)
Group 1 stockholders’ equity	1,247,508	1,077,146
Intercompany note receivable	0	0
Total stockholders’ equity	1,247,508	1,077,146
Total liabilities and stockholders' equity	3,028,329	2,787,844
Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	10,096	8,039	6,338	25,379
Contracts-in-transit and vehicle receivables, net	266,788	241,097
Accounts and notes receivable, net	144,872	140,985
Intercompany accounts receivable	12,948	8,929
Inventory, Net	1,434,852	1,386,871
Prepaid expenses and other current assets	8,378	7,188
Total current assets	1,877,934	1,793,109
PROPERTY AND EQUIPMENT, net	1,121,108	990,084
GOODWILL	835,268	805,935
INTANGIBLE FRANCHISE RIGHTS	255,980	260,534
INVESTMENT IN SUBSIDIARIES	0	0
OTHER ASSETS	13,682	19,313
Total assets	4,103,972	3,868,975
Floorplan notes payable - credit facility and other	1,219,844	1,131,718
Offset account related to floorplan notes payable - credit facility	(86,547)	(59,626)
Floorplan notes payable - manufacturer affiliates	272,563	281,747
FMCC Offset	(22,500)	(25,500)
Current maturities of long-term debt and short-term financing	31,229	44,659
Derivative Liability, Current	1,996	3,941
Accounts payable	229,470	211,050
Intercompany accounts payable	0	0
Accrued expenses	150,241	156,648
Total current liabilities	1,796,296	1,744,637
LONG-TERM DEBT, net of current maturities	360,526	324,540
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	8,583	20,470
DEFERRED INCOME TAXES AND OTHER LIABILITIES	210,216	240,348
Group 1 stockholders’ equity	2,619,346	2,414,642
Intercompany note receivable	(890,995)	(875,662)
Total stockholders’ equity	1,728,351	1,538,980
Total liabilities and stockholders' equity	4,103,972	3,868,975
Non-Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	18,691	12,953	$ 6,699	$ 15,596
Contracts-in-transit and vehicle receivables, net	39,645	28,411
Accounts and notes receivable, net	43,739	32,379
Intercompany accounts receivable	0	0
Inventory, Net	328,441	264,944
Prepaid expenses and other current assets	31,750	27,204
Total current assets	462,266	365,891
PROPERTY AND EQUIPMENT, net	197,851	135,799
GOODWILL	77,766	70,828
INTANGIBLE FRANCHISE RIGHTS	29,652	24,342
INVESTMENT IN SUBSIDIARIES	0	0
OTHER ASSETS	10,572	4,481
Total assets	778,107	601,341
Floorplan notes payable - credit facility and other	20,851	4,936
Offset account related to floorplan notes payable - credit facility	0	0
Floorplan notes payable - manufacturer affiliates	124,620	110,914
FMCC Offset	0	0
Current maturities of long-term debt and short-term financing	21,639	27,760
Derivative Liability, Current	0	0
Accounts payable	183,511	145,049
Intercompany accounts payable	39,936	8,929
Accrued expenses	26,829	19,821
Total current liabilities	417,386	317,409
LONG-TERM DEBT, net of current maturities	92,456	52,213
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0	0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	11,430	5,979
Group 1 stockholders’ equity	256,835	225,740
Intercompany note receivable	0	0
Total stockholders’ equity	256,835	225,740
Total liabilities and stockholders' equity	$ 778,107	$ 601,341