Condensed Consolidating Financial Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
CURRENT ASSETS:
Cash and cash equivalents	$ 32,027		$ 28,787
Contracts-in-transit and vehicle receivables, net	235,609		306,433
Accounts and notes receivable, net	169,318	$ 200,234	188,611
Intercompany accounts receivable	0		0
Inventories, net	1,733,756	1,759,633	1,763,293
Prepaid expenses and other current assets	77,996	50,745	42,062
Total current assets	2,248,706		2,329,186
PROPERTY AND EQUIPMENT, net	1,350,929		1,318,959
GOODWILL	968,771		913,034
INTANGIBLE FRANCHISE RIGHTS	276,285		285,632
INVESTMENT IN SUBSIDIARIES	0		0
OTHER ASSETS	36,175		24,254
Total assets	4,880,866		4,871,065
CURRENT LIABILITIES:
Floorplan notes payable — credit facility and other	1,155,034		1,240,695
Offset account related to floorplan notes payable - credit facility	(71,397)		(86,547)
Floorplan notes payable — manufacturer affiliates	415,615		397,183
Offset account related to floorplan notes payable - manufacturer affiliates	(20,500)		(22,500)
Current maturities of long-term debt and short-term financing	76,080		77,609
Current liabilities from interest rate risk management activities	311		1,996
Accounts payable	428,441	$ 414,737	412,981
Intercompany accounts payable	0		0
Accrued expenses	207,082		177,070
Total current liabilities	2,190,666		2,198,487
LONG-TERM DEBT, net of current maturities	1,304,127		1,318,184
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	444		8,583
DEFERRED INCOME TAXES AND OTHER LIABILITIES	237,283		221,529
STOCKHOLDERS’ EQUITY:
Group 1 stockholders’ equity	1,148,346		1,124,282
Intercompany note receivable	0		0
Total stockholders’ equity	1,148,346		1,124,282
Total liabilities and stockholders’ equity	4,880,866		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	(83,744)		(39,936)
Inventories, net	0		0
Prepaid expenses and other current assets	0		0
Total current assets	(83,744)		(39,936)
PROPERTY AND EQUIPMENT, net	0		0
GOODWILL	0		0
INTANGIBLE FRANCHISE RIGHTS	0		0
INVESTMENT IN SUBSIDIARIES	(3,194,823)		(2,999,407)
OTHER ASSETS	0		0
Total assets	(3,278,567)		(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	(1,139,441)		(930,931)
Accrued expenses	0		0
Total current liabilities	(1,139,441)		(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	(3,194,823)		(2,999,407)
Intercompany note receivable	1,055,697		890,995
Total stockholders’ equity	(2,139,126)		(2,108,412)
Total liabilities and stockholders’ equity	(3,278,567)		(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	32,585		26,988
Inventories, net	0		0
Prepaid expenses and other current assets	214		1,934
Total current assets	32,799		28,922
PROPERTY AND EQUIPMENT, net	0		0
GOODWILL	0		0
INTANGIBLE FRANCHISE RIGHTS	0		0
INVESTMENT IN SUBSIDIARIES	3,194,823		2,999,407
OTHER ASSETS	0		0
Total assets	3,227,622		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	1,088,282		890,995
Accrued expenses	0		0
Total current liabilities	1,088,282		915,736
LONG-TERM DEBT, net of current maturities	872,477		865,202
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0		0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	1,148		(117)
STOCKHOLDERS’ EQUITY:
Group 1 stockholders’ equity	1,265,715		1,247,508
Intercompany note receivable	0		0
Total stockholders’ equity	1,265,715		1,247,508
Total liabilities and stockholders’ equity	3,227,622		3,028,329
Guarantor Subsidiaries
CURRENT ASSETS:
Cash and cash equivalents	9,062		10,096
Contracts-in-transit and vehicle receivables, net	169,199		266,788
Accounts and notes receivable, net	133,506		144,872
Intercompany accounts receivable	51,159		12,948
Inventories, net	1,392,370		1,434,852
Prepaid expenses and other current assets	23,953		8,378
Total current assets	1,779,249		1,877,934
PROPERTY AND EQUIPMENT, net	1,125,421		1,121,108
GOODWILL	865,062		835,268
INTANGIBLE FRANCHISE RIGHTS	239,738		255,980
INVESTMENT IN SUBSIDIARIES	0		0
OTHER ASSETS	25,028		13,682
Total assets	4,034,498		4,103,972
CURRENT LIABILITIES:
Floorplan notes payable — credit facility and other	1,126,477		1,219,844
Offset account related to floorplan notes payable - credit facility	(71,397)		(86,547)
Floorplan notes payable — manufacturer affiliates	259,697		272,563
Offset account related to floorplan notes payable - manufacturer affiliates	(20,500)		(22,500)
Current maturities of long-term debt and short-term financing	63,940		31,229
Current liabilities from interest rate risk management activities	311		1,996
Accounts payable	202,823		229,470
Intercompany accounts payable	0		0
Accrued expenses	170,607		150,241
Total current liabilities	1,731,958		1,796,296
LONG-TERM DEBT, net of current maturities	311,592		360,526
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	444		8,583
DEFERRED INCOME TAXES AND OTHER LIABILITIES	224,405		210,216
STOCKHOLDERS’ EQUITY:
Group 1 stockholders’ equity	2,821,796		2,619,346
Intercompany note receivable	(1,055,697)		(890,995)
Total stockholders’ equity	1,766,099		1,728,351
Total liabilities and stockholders’ equity	4,034,498		4,103,972
Non-Guarantor Subsidiaries
CURRENT ASSETS:
Cash and cash equivalents	22,965		18,691
Contracts-in-transit and vehicle receivables, net	66,410		39,645
Accounts and notes receivable, net	35,812		43,739
Intercompany accounts receivable	0		0
Inventories, net	341,386		328,441
Prepaid expenses and other current assets	53,829		31,750
Total current assets	520,402		462,266
PROPERTY AND EQUIPMENT, net	225,508		197,851
GOODWILL	103,709		77,766
INTANGIBLE FRANCHISE RIGHTS	36,547		29,652
INVESTMENT IN SUBSIDIARIES	0		0
OTHER ASSETS	11,147		10,572
Total assets	897,313		778,107
CURRENT LIABILITIES:
Floorplan notes payable — credit facility and other	28,557		20,851
Offset account related to floorplan notes payable - credit facility	0		0
Floorplan notes payable — manufacturer affiliates	155,918		124,620
Offset account related to floorplan notes payable - manufacturer affiliates	0		0
Current maturities of long-term debt and short-term financing	12,140		21,639
Current liabilities from interest rate risk management activities	0		0
Accounts payable	225,618		183,511
Intercompany accounts payable	51,159		39,936
Accrued expenses	36,475		26,829
Total current liabilities	509,867		417,386
LONG-TERM DEBT, net of current maturities	120,058		92,456
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0		0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	11,730		11,430
STOCKHOLDERS’ EQUITY:
Group 1 stockholders’ equity	255,658		256,835
Intercompany note receivable	0		0
Total stockholders’ equity	255,658		256,835
Total liabilities and stockholders’ equity	$ 897,313		$ 778,107