Condensed Consolidated Financial Information Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2018		Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	$ 33,090			$ 28,787		$ 24,246
Contracts-in-transit and vehicle receivables, net	297,885			306,433
Accounts and notes receivable, net	200,159		$ 208,917	188,611
Intercompany accounts receivable	0			0
Inventories, net	1,804,176		1,759,633	1,763,293
Prepaid expenses and other current assets	77,888			42,062
Total current assets	2,413,198			2,329,186
PROPERTY AND EQUIPMENT, net	1,357,870			1,318,959
GOODWILL	960,110	[1]		913,034
INTANGIBLE FRANCHISE RIGHTS	295,626			285,632
INVESTMENT IN SUBSIDIARIES	0			0
OTHER ASSETS	31,674			24,254
Total assets	5,058,478			4,871,065
Floorplan notes payable — credit facility and other	1,199,741			1,240,695
Offset account related to floorplan notes payable - credit facility	(74,359)			(86,547)
Floorplan notes payable - manufacturer affiliates	420,315			397,183
Offset account related to floorplan notes payable - manufacturer affiliates	(24,000)			(22,500)
Current maturities of long-term debt and short-term financing	58,195			77,609
Derivative Liability, Current	1,177			1,996
Accounts payable	(491,841)		$ (414,737)	(412,981)
Intercompany accounts payable	0			0
Accrued Liabilities, Current	194,248			177,070
Total current liabilities	2,267,158			2,198,487
LONG-TERM DEBT, net of current maturities	1,376,770			1,318,184
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	3,512			8,583
DEFERRED INCOME TAXES AND OTHER LIABILITIES	232,959			221,529
Group 1 stockholders’ equity	1,178,079			1,124,282
Intercompany note receivable	0			0
Total stockholders’ equity	1,178,079			1,124,282
Total liabilities and stockholders’ equity	5,058,478			4,871,065
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	28,035			26,988
Inventories, net	0			0
Prepaid expenses and other current assets	0			1,934
Total current assets	28,035			28,922
PROPERTY AND EQUIPMENT, net	0			0
GOODWILL	0			0
INTANGIBLE FRANCHISE RIGHTS	0			0
INVESTMENT IN SUBSIDIARIES	3,017,378			2,999,407
OTHER ASSETS	0			0
Total assets	3,045,413			3,028,329
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 and short-term financing	0			24,741
Derivative Liability, Current	0			0
Accounts payable	0			0
Intercompany accounts payable	904,425			890,995
Accrued Liabilities, Current	696			0
Total current liabilities	905,121			915,736
LONG-TERM DEBT, net of current maturities	866,803			865,202
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0			0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	855			(117)
Group 1 stockholders’ equity	1,272,634			1,247,508
Intercompany note receivable	0			0
Total stockholders’ equity	1,272,634			1,247,508
Total liabilities and stockholders’ equity	3,045,413			3,028,329
Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	8,874			10,096
Contracts-in-transit and vehicle receivables, net	209,364			266,788
Accounts and notes receivable, net	140,243			144,872
Intercompany accounts receivable	40,271			12,948
Inventories, net	1,438,626			1,434,852
Prepaid expenses and other current assets	28,151			8,378
Total current assets	1,865,529			1,877,934
PROPERTY AND EQUIPMENT, net	1,127,632			1,121,108
GOODWILL	849,485			835,268
INTANGIBLE FRANCHISE RIGHTS	257,281			255,980
INVESTMENT IN SUBSIDIARIES	0			0
OTHER ASSETS	19,943			13,682
Total assets	4,119,870			4,103,972
Floorplan notes payable — credit facility and other	1,161,401			1,219,844
Offset account related to floorplan notes payable - credit facility	(74,359)			(86,547)
Floorplan notes payable - manufacturer affiliates	286,320			272,563
Offset account related to floorplan notes payable - manufacturer affiliates	(24,000)			(22,500)
Current maturities of long-term debt and short-term financing	46,207			31,229
Derivative Liability, Current	1,177			1,996
Accounts payable	(217,150)			(229,470)
Intercompany accounts payable	0			0
Accrued Liabilities, Current	159,784			150,241
Total current liabilities	1,773,680			1,796,296
LONG-TERM DEBT, net of current maturities	373,977			360,526
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	3,512			8,583
DEFERRED INCOME TAXES AND OTHER LIABILITIES	218,811			210,216
Group 1 stockholders’ equity	2,654,315			2,619,346
Intercompany note receivable	(904,425)			890,995
Total stockholders’ equity	1,749,890			1,728,351
Total liabilities and stockholders’ equity	4,119,870			4,103,972
Non-Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	24,216			18,691
Contracts-in-transit and vehicle receivables, net	88,521			39,645
Accounts and notes receivable, net	59,916			43,739
Intercompany accounts receivable	0			0
Inventories, net	365,550			328,441
Prepaid expenses and other current assets	49,737			31,750
Total current assets	587,940			462,266
PROPERTY AND EQUIPMENT, net	230,238			197,851
GOODWILL	110,625			77,766
INTANGIBLE FRANCHISE RIGHTS	38,345			29,652
INVESTMENT IN SUBSIDIARIES	0			0
OTHER ASSETS	11,731			10,572
Total assets	978,879			778,107
Floorplan notes payable — credit facility and other	38,340			20,851
Offset account related to floorplan notes payable - credit facility	0			0
Floorplan notes payable - manufacturer affiliates	133,995			124,620
Offset account related to floorplan notes payable - manufacturer affiliates	0			0
Current maturities of long-term debt and short-term financing	11,988			21,639
Derivative Liability, Current	0			0
Accounts payable	(274,691)			(183,511)
Intercompany accounts payable	68,306			39,936
Accrued Liabilities, Current	33,768			26,829
Total current liabilities	561,088			417,386
LONG-TERM DEBT, net of current maturities	135,990			92,456
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0			0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	13,293			11,430
Group 1 stockholders’ equity	268,508			256,835
Intercompany note receivable	0			0
Total stockholders’ equity	268,508			256,835
Total liabilities and stockholders’ equity	978,879			778,107
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	(68,306)			(39,936)
Inventories, net	0			0
Prepaid expenses and other current assets	0			0
Total current assets	(68,306)			(39,936)
PROPERTY AND EQUIPMENT, net	0			0
GOODWILL	0			0
INTANGIBLE FRANCHISE RIGHTS	0			0
INVESTMENT IN SUBSIDIARIES	(3,017,378)			(2,999,407)
OTHER ASSETS	0			0
Total assets	(3,085,684)			(3,039,343)
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 and short-term financing	0			0
Derivative Liability, Current	0			0
Accounts payable	0			0
Intercompany accounts payable	(972,731)			(930,931)
Accrued Liabilities, Current	0			0
Total current liabilities	(972,731)			(930,931)
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	(3,017,378)			(2,999,407)
Intercompany note receivable	904,425			(890,995)
Total stockholders’ equity	(2,112,953)			(2,108,412)
Total liabilities and stockholders’ equity	$ (3,085,684)			$ (3,039,343)

[1] (1) Net of accumulated impairment of $97.8 million