Condensed Consolidating Financial Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
CURRENT ASSETS:
Cash and cash equivalents	$ 41,575		$ 28,787
Contracts-in-transit and vehicle receivables, net	249,706		306,433
Accounts and notes receivable, net	178,339	$ 200,234	188,611
Intercompany accounts receivable	0		0
Inventories, net	1,721,249	1,759,633	1,763,293
Prepaid expenses and other current assets	80,957	50,745	42,062
Total current assets	2,271,826		2,329,186
PROPERTY AND EQUIPMENT, net	1,348,521		1,318,959
GOODWILL	945,835		913,034
INTANGIBLE FRANCHISE RIGHTS	287,366		285,632
INVESTMENT IN SUBSIDIARIES	0		0
OTHER ASSETS	33,194		24,254
Total assets	4,886,742		4,871,065
CURRENT LIABILITIES:
Floorplan notes payable — credit facility and other	1,147,892		1,240,695
Offset account related to floorplan notes payable - credit facility	(119,562)		(86,547)
Floorplan notes payable — manufacturer affiliates	404,233		397,183
Offset account related to floorplan notes payable - manufacturer affiliates	(24,500)		(22,500)
Current maturities of long-term debt and short-term financing	76,412		77,609
Current liabilities from interest rate risk management activities	682		1,996
Accounts payable	442,577	$ 414,737	412,981
Intercompany accounts payable	0		0
Accrued expenses	189,027		177,070
Total current liabilities	2,116,761		2,198,487
LONG-TERM DEBT, net of current maturities	1,357,998		1,318,184
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	1,597		8,583
DEFERRED INCOME TAXES AND OTHER LIABILITIES	237,774		221,529
STOCKHOLDERS’ EQUITY:
Group 1 stockholders’ equity	1,172,612		1,124,282
Intercompany note receivable	0		0
Total stockholders’ equity	1,172,612		1,124,282
Total liabilities and stockholders’ equity	4,886,742		4,871,065
Elimination
CURRENT ASSETS:
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	(96,071)		(39,936)
Inventories, net	0		0
Prepaid expenses and other current assets	0		0
Total current assets	(96,071)		(39,936)
PROPERTY AND EQUIPMENT, net	0		0
GOODWILL	0		0
INTANGIBLE FRANCHISE RIGHTS	0		0
INVESTMENT IN SUBSIDIARIES	(2,982,451)		(2,999,407)
OTHER ASSETS	0		0
Total assets	(3,078,522)		(3,039,343)
CURRENT LIABILITIES:
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
Current liabilities from interest rate risk management activities	0		0
Accounts payable	0		0
Intercompany accounts payable	(911,991)		(930,931)
Accrued expenses	0		0
Total current liabilities	(911,991)		(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
STOCKHOLDERS’ EQUITY:
Group 1 stockholders’ equity	(2,982,451)		(2,999,407)
Intercompany note receivable	815,920		890,995
Total stockholders’ equity	(2,166,531)		(2,108,412)
Total liabilities and stockholders’ equity	(3,078,522)		(3,039,343)
Group 1 Automotive, Inc.
CURRENT ASSETS:
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	39,636		26,988
Inventories, net	0		0
Prepaid expenses and other current assets	779		1,934
Total current assets	40,415		28,922
PROPERTY AND EQUIPMENT, net	0		0
GOODWILL	0		0
INTANGIBLE FRANCHISE RIGHTS	0		0
INVESTMENT IN SUBSIDIARIES	2,982,451		2,999,407
OTHER ASSETS	0		0
Total assets	3,022,866		3,028,329
CURRENT LIABILITIES:
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
Current liabilities from interest rate risk management activities	0		0
Accounts payable	0		0
Intercompany accounts payable	855,557		890,995
Accrued expenses	0		0
Total current liabilities	855,557		915,736
LONG-TERM DEBT, net of current maturities	878,964		865,202
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0		0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	769		(117)
STOCKHOLDERS’ EQUITY:
Group 1 stockholders’ equity	1,287,576		1,247,508
Intercompany note receivable	0		0
Total stockholders’ equity	1,287,576		1,247,508
Total liabilities and stockholders’ equity	3,022,866		3,028,329
Guarantor Subsidiaries
CURRENT ASSETS:
Cash and cash equivalents	8,676		10,096
Contracts-in-transit and vehicle receivables, net	188,592		266,788
Accounts and notes receivable, net	132,452		144,872
Intercompany accounts receivable	56,435		12,948
Inventories, net	1,379,275		1,434,852
Prepaid expenses and other current assets	35,273		8,378
Total current assets	1,800,703		1,877,934
PROPERTY AND EQUIPMENT, net	1,118,917		1,121,108
GOODWILL	839,498		835,268
INTANGIBLE FRANCHISE RIGHTS	251,240		255,980
INVESTMENT IN SUBSIDIARIES	0		0
OTHER ASSETS	22,243		13,682
Total assets	4,032,601		4,103,972
CURRENT LIABILITIES:
Floorplan notes payable — credit facility and other	1,115,163		1,219,844
Offset account related to floorplan notes payable - credit facility	(119,562)		(86,547)
Floorplan notes payable — manufacturer affiliates	277,299		272,563
Offset account related to floorplan notes payable - manufacturer affiliates	(24,500)		(22,500)
Current maturities of long-term debt and short-term financing	50,720		31,229
Current liabilities from interest rate risk management activities	682		1,996
Accounts payable	210,791		229,470
Intercompany accounts payable	0		0
Accrued expenses	156,283		150,241
Total current liabilities	1,666,876		1,796,296
LONG-TERM DEBT, net of current maturities	354,002		360,526
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	1,597		8,583
DEFERRED INCOME TAXES AND OTHER LIABILITIES	223,692		210,216
STOCKHOLDERS’ EQUITY:
Group 1 stockholders’ equity	2,602,354		2,619,346
Intercompany note receivable	(815,920)		(890,995)
Total stockholders’ equity	1,786,434		1,728,351
Total liabilities and stockholders’ equity	4,032,601		4,103,972
Non-Guarantor Subsidiaries
CURRENT ASSETS:
Cash and cash equivalents	32,899		18,691
Contracts-in-transit and vehicle receivables, net	61,114		39,645
Accounts and notes receivable, net	45,887		43,739
Intercompany accounts receivable	0		0
Inventories, net	341,974		328,441
Prepaid expenses and other current assets	44,905		31,750
Total current assets	526,779		462,266
PROPERTY AND EQUIPMENT, net	229,604		197,851
GOODWILL	106,337		77,766
INTANGIBLE FRANCHISE RIGHTS	36,126		29,652
INVESTMENT IN SUBSIDIARIES	0		0
OTHER ASSETS	10,951		10,572
Total assets	909,797		778,107
CURRENT LIABILITIES:
Floorplan notes payable — credit facility and other	32,729		20,851
Offset account related to floorplan notes payable - credit facility	0		0
Floorplan notes payable — manufacturer affiliates	126,934		124,620
Offset account related to floorplan notes payable - manufacturer affiliates	0		0
Current maturities of long-term debt and short-term financing	25,692		21,639
Current liabilities from interest rate risk management activities	0		0
Accounts payable	231,786		183,511
Intercompany accounts payable	56,434		39,936
Accrued expenses	32,744		26,829
Total current liabilities	506,319		417,386
LONG-TERM DEBT, net of current maturities	125,032		92,456
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0		0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	13,313		11,430
STOCKHOLDERS’ EQUITY:
Group 1 stockholders’ equity	265,133		256,835
Intercompany note receivable	0		0
Total stockholders’ equity	265,133		256,835
Total liabilities and stockholders’ equity	$ 909,797		$ 778,107