Condensed Consolidated Financial Information Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2017		Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	$ 26,573		$ 20,992	$ 24,965	$ 13,037
Contracts-in-transit and vehicle receivables, net	217,378		269,508
Accounts and notes receivable, net	158,542		173,364
Intercompany accounts receivable	0		0
Inventories, net	1,803,909		1,651,815
Prepaid expenses and other current assets	87,470		34,908
Total current assets	2,293,872		2,150,587
PROPERTY AND EQUIPMENT, net	1,167,345		1,125,883
GOODWILL	878,776	[1]	876,763
INTANGIBLE FRANCHISE RIGHTS	285,689		284,876
INVESTMENT IN SUBSIDIARIES	0		0
OTHER ASSETS	18,903		23,794
Total assets	4,644,585		4,461,903
Floorplan notes payable — credit facility and other	1,214,084		1,136,654
Offset account related to floorplan notes payable - credit facility	(55,579)		(59,626)
Floorplan notes payable - manufacturer affiliates	430,619		392,661
Offset account related to floorplan notes payable - manufacturer affiliates	(21,000)		(25,500)
Current maturities of long-term debt and short-term financing	53,184		72,419
Derivative Liability, Current	1,626		3,941
Accounts payable	330,747		356,099
Intercompany accounts payable	0		0
Accrued expenses	173,173		176,469
Total current liabilities	2,126,854		2,053,117
LONG-TERM DEBT, net of current maturities	1,263,845		1,212,809
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	18,065		20,470
DEFERRED INCOME TAXES AND OTHER LIABILITIES	264,173		245,307
Group 1 stockholders’ equity	971,648		930,200
Intercompany note receivable	0		0
Total stockholders’ equity	971,648		930,200
Total liabilities and stockholders’ equity	4,644,585		4,461,903
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	32,509		0
Inventories, net	0		0
Prepaid expenses and other current assets	477		516
Total current assets	32,986		516
PROPERTY AND EQUIPMENT, net	0		0
GOODWILL	0		0
INTANGIBLE FRANCHISE RIGHTS	0		0
INVESTMENT IN SUBSIDIARIES	2,878,385		2,787,328
OTHER ASSETS	0		0
Total assets	2,911,371		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
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	934,759
Accrued expenses	0		0
Total current liabilities	934,759		875,662
LONG-TERM DEBT, net of current maturities	869,633
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0		0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	(937)
Group 1 stockholders’ equity	1,107,916
Intercompany note receivable	0		0
Total stockholders’ equity	1,107,916		1,077,146
Total liabilities and stockholders’ equity	2,911,371		2,787,844
Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	5,878		8,039	5,495	6,338
Contracts-in-transit and vehicle receivables, net	165,502		241,097
Accounts and notes receivable, net	125,375		140,985
Intercompany accounts receivable	10,115		8,929
Inventories, net	1,539,571		1,386,871
Prepaid expenses and other current assets	7,152		7,188
Total current assets	1,853,593		1,793,109
PROPERTY AND EQUIPMENT, net	1,023,669		990,084
GOODWILL	805,924		805,935
INTANGIBLE FRANCHISE RIGHTS	260,534		260,534
INVESTMENT IN SUBSIDIARIES	0		0
OTHER ASSETS	13,470		19,313
Total assets	3,957,190		3,868,975
Floorplan notes payable — credit facility and other	1,202,663		1,131,718
Offset account related to floorplan notes payable - credit facility	(55,579)		(59,626)
Floorplan notes payable - manufacturer affiliates	297,651		281,747
Offset account related to floorplan notes payable - manufacturer affiliates	(21,000)		(25,500)
Current maturities of long-term debt and short-term financing	33,978		44,659
Derivative Liability, Current	1,626		3,941
Accounts payable	185,036		211,050
Intercompany accounts payable	0		0
Accrued expenses	145,092		156,648
Total current liabilities	1,789,467		1,744,637
LONG-TERM DEBT, net of current maturities	323,769		324,540
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	18,065		20,470
DEFERRED INCOME TAXES AND OTHER LIABILITIES	259,357		240,348
Group 1 stockholders’ equity	2,501,291		2,414,642
Intercompany note receivable	(934,759)		(875,662)
Total stockholders’ equity	1,566,532		1,538,980
Total liabilities and stockholders’ equity	3,957,190		3,868,975
Non-Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	20,695		12,953	$ 19,470	$ 6,699
Contracts-in-transit and vehicle receivables, net	51,876		28,411
Accounts and notes receivable, net	33,167		32,379
Intercompany accounts receivable	0		0
Inventories, net	264,338		264,944
Prepaid expenses and other current assets	79,841		27,204
Total current assets	449,917		365,891
PROPERTY AND EQUIPMENT, net	143,676		135,799
GOODWILL	72,852		70,828
INTANGIBLE FRANCHISE RIGHTS	25,155		24,342
INVESTMENT IN SUBSIDIARIES	0		0
OTHER ASSETS	5,433		4,481
Total assets	697,033		601,341
Floorplan notes payable — credit facility and other	11,421		4,936
Offset account related to floorplan notes payable - credit facility	0		0
Floorplan notes payable - manufacturer affiliates	132,968		110,914
Offset account related to floorplan notes payable - manufacturer affiliates	0		0
Current maturities of long-term debt and short-term financing	19,206		27,760
Derivative Liability, Current	0		0
Accounts payable	145,711		145,049
Intercompany accounts payable	42,624		8,929
Accrued expenses	28,081		19,821
Total current liabilities	380,011		317,409
LONG-TERM DEBT, net of current maturities	70,443		52,213
LIABILITIES FROM INTEREST RATE RISK MANAGEMENT ACTIVITIES	0		0
DEFERRED INCOME TAXES AND OTHER LIABILITIES	5,753		5,979
Group 1 stockholders’ equity	240,826		225,740
Intercompany note receivable	0		0
Total stockholders’ equity	240,826		225,740
Total liabilities and stockholders’ equity	697,033		601,341
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	(42,624)		(8,929)
Inventories, net	0		0
Prepaid expenses and other current assets	0		0
Total current assets	(42,624)		(8,929)
PROPERTY AND EQUIPMENT, net	0		0
GOODWILL	0		0
INTANGIBLE FRANCHISE RIGHTS	0		0
INVESTMENT IN SUBSIDIARIES	(2,878,385)		(2,787,328)
OTHER ASSETS	0		0
Total assets	(2,921,009)		(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
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	(977,383)		(884,591)
Accrued expenses	0		0
Total current liabilities	(977,383)		(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	(2,878,385)		(2,787,328)
Intercompany note receivable	934,759		875,662
Total stockholders’ equity	(1,943,626)		(1,911,666)
Total liabilities and stockholders’ equity	$ (2,921,009)		$ (2,796,257)

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