Condensed Consolidated Financial Information Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	$ 21,968	$ 40,975	$ 50,652	$ 20,215
Contracts-in-transit and vehicle receivables, net	205,636	237,448
Accounts and notes receivable, net	147,110	151,330
Intercompany accounts receivable	0	0
Inventories, net	1,634,421	1,556,705
Deferred income taxes	13,099	11,062
Prepaid expenses and other current assets	22,556	37,699
Total current assets	2,044,790	2,035,219
PROPERTY AND EQUIPMENT, net	978,892	950,388
GOODWILL	912,639	830,377
INTANGIBLE FRANCHISE RIGHTS	339,763	303,947
INVESTMENT IN SUBSIDIARIES	0	0
OTHER ASSETS	15,966	21,561
Total assets	4,292,050	4,141,492
Floorplan notes payable — credit facility and other	1,177,513	1,143,246
Offset account related to floorplan notes payable - credit facility	(28,073)	(39,616)
Floorplan notes payable - manufacturer affiliates	347,657	307,656
Offset account related to floorplan notes payable - manufacturer affiliates	(20,001)	(22,500)
Current maturities of long-term debt	55,959	72,630
Accounts payable	290,509	288,320
Intercompany accounts payable	0	0
Accrued expenses	158,283	172,463
Total current liabilities	1,981,847	1,922,199
LONG-TERM DEBT, net of current maturities	1,084,161	1,008,837
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	39,724	25,311
DEFERRED INCOME TAXES AND OTHER LIABILITIES	216,990	207,135
Group 1 stockholders’ equity	969,328	978,010
Intercompany note receivable	0	0
Total stockholders’ equity	969,328	978,010
Total liabilities and stockholders’ equity	4,292,050	4,141,492
Parent Company [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	0	0	26	0
Contracts-in-transit and vehicle receivables, net	0	0
Accounts and notes receivable, net	0	0
Intercompany accounts receivable	0	0
Inventories, net	0	0
Deferred income taxes	288	196
Prepaid expenses and other current assets	785	590
Total current assets	1,073	786
PROPERTY AND EQUIPMENT, net	0	0
GOODWILL	0	0
INTANGIBLE FRANCHISE RIGHTS	0	0
INVESTMENT IN SUBSIDIARIES	2,162,133	1,964,442
OTHER ASSETS	0	0
Total assets	2,163,206	1,965,228
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
Offset account related to floorplan notes payable - manufacturer affiliates	0	0
Current maturities of long-term debt	0	0
Accounts payable	0	0
Intercompany accounts payable	368,915	295,421
Accrued expenses	0	0
Total current liabilities	368,915	295,421
LONG-TERM DEBT, net of current maturities	679,158	609,812
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0	0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	0	0
Group 1 stockholders’ equity	1,115,133	1,059,995
Intercompany note receivable	0	0
Total stockholders’ equity	1,115,133	1,059,995
Total liabilities and stockholders’ equity	2,163,206	1,965,228
Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	4,397	25,379	34,165	4,306
Contracts-in-transit and vehicle receivables, net	173,859	218,361
Accounts and notes receivable, net	118,137	117,427
Intercompany accounts receivable	263,906	276,217
Inventories, net	1,438,310	1,342,022
Deferred income taxes	12,811	10,866
Prepaid expenses and other current assets	8,000	22,039
Total current assets	2,019,420	2,012,311
PROPERTY AND EQUIPMENT, net	863,526	839,063
GOODWILL	814,759	700,642
INTANGIBLE FRANCHISE RIGHTS	306,745	257,502
INVESTMENT IN SUBSIDIARIES	0	0
OTHER ASSETS	8,683	10,120
Total assets	4,013,133	3,819,638
Floorplan notes payable — credit facility and other	1,176,385	1,137,743
Offset account related to floorplan notes payable - credit facility	(28,073)	(39,616)
Floorplan notes payable - manufacturer affiliates	253,361	207,329
Offset account related to floorplan notes payable - manufacturer affiliates	(20,001)	(22,500)
Current maturities of long-term debt	50,756	61,185
Accounts payable	177,270	176,143
Intercompany accounts payable	0	0
Accrued expenses	137,675	149,700
Total current liabilities	1,747,373	1,669,984
LONG-TERM DEBT, net of current maturities	337,858	347,202
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	39,724	25,311
DEFERRED INCOME TAXES AND OTHER LIABILITIES	206,938	193,077
Group 1 stockholders’ equity	2,050,155	1,879,485
Intercompany note receivable	(368,915)	(295,421)
Total stockholders’ equity	1,681,240	1,584,064
Total liabilities and stockholders’ equity	4,013,133	3,819,638
Non-Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	17,571	15,596	$ 16,461	$ 15,909
Contracts-in-transit and vehicle receivables, net	31,777	19,087
Accounts and notes receivable, net	28,973	33,903
Intercompany accounts receivable	0	0
Inventories, net	196,111	214,683
Deferred income taxes	0	0
Prepaid expenses and other current assets	13,771	15,070
Total current assets	288,203	298,339
PROPERTY AND EQUIPMENT, net	115,366	111,325
GOODWILL	97,880	129,735
INTANGIBLE FRANCHISE RIGHTS	33,018	46,445
INVESTMENT IN SUBSIDIARIES	0	0
OTHER ASSETS	7,283	11,441
Total assets	541,750	597,285
Floorplan notes payable — credit facility and other	1,128	5,503
Offset account related to floorplan notes payable - credit facility	0	0
Floorplan notes payable - manufacturer affiliates	94,296	100,327
Offset account related to floorplan notes payable - manufacturer affiliates	0	0
Current maturities of long-term debt	5,203	11,445
Accounts payable	113,239	112,177
Intercompany accounts payable	263,906	276,217
Accrued expenses	20,608	22,763
Total current liabilities	498,380	528,432
LONG-TERM DEBT, net of current maturities	67,145	51,823
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0	0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	10,052	14,058
Group 1 stockholders’ equity	(33,827)	2,972
Intercompany note receivable	0	0
Total stockholders’ equity	(33,827)	2,972
Total liabilities and stockholders’ equity	541,750	597,285
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	(263,906)	(276,217)
Inventories, net	0	0
Deferred income taxes	0	0
Prepaid expenses and other current assets	0	0
Total current assets	(263,906)	(276,217)
PROPERTY AND EQUIPMENT, net	0	0
GOODWILL	0	0
INTANGIBLE FRANCHISE RIGHTS	0	0
INVESTMENT IN SUBSIDIARIES	(2,162,133)	(1,964,442)
OTHER ASSETS	0	0
Total assets	(2,426,039)	(2,240,659)
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
Offset account related to floorplan notes payable - manufacturer affiliates	0	0
Current maturities of long-term debt	0	0
Accounts payable	0	0
Intercompany accounts payable	(632,821)	(571,638)
Accrued expenses	0	0
Total current liabilities	(632,821)	(571,638)
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	(2,162,133)	(1,964,442)
Intercompany note receivable	368,915	295,421
Total stockholders’ equity	(1,793,218)	(1,669,021)
Total liabilities and stockholders’ equity	$ (2,426,039)	$ (2,240,659)