Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
New Vehicle [Member]
Revenues:
Total revenues	$ 1,297,511	$ 1,227,080	$ 3,602,603	$ 3,384,408
Cost of sales:
Total cost of sales	1,221,668	1,149,923	3,387,132	3,176,135
Used Retail Vehicle [Member]
Revenues:
Total revenues	580,885	505,885	1,667,258	1,457,617
Cost of sales:
Total cost of sales	512,076	443,598	1,466,947	1,273,195
Used Wholesale Vehicle [Member]
Revenues:
Total revenues	75,271	69,472	207,131	198,476
Cost of sales:
Total cost of sales	74,353	68,892	202,897	194,329
Finance and Insurance [Member]
Revenues:
Total revenues	87,709	76,633	246,390	213,700
Service, Body and Parts [Member]
Revenues:
Total revenues	217,148	189,796	616,088	545,966
Cost of sales:
Total cost of sales	112,806	95,846	317,028	276,828
Fleet and Other [Member]
Revenues:
Total revenues	11,443	15,979	46,697	70,803
Cost of sales:
Total cost of sales	11,803	15,399	45,684	68,272
Total revenues	2,269,967	2,084,845	6,386,167	5,870,970
Total cost of sales	1,932,706	1,773,658	5,419,688	4,988,759
Gross profit	337,261	311,187	966,479	882,211
Asset impairments	3,498	4,131	10,494	14,391
Selling, general and administrative	228,134	223,728	662,766	610,956
Depreciation and amortization	12,206	10,531	36,372	30,544
Operating income	93,423	72,797	256,847	226,320
Floor plan interest expense	(6,186)	(4,951)	(18,304)	(14,255)
Other interest expense, net	(5,647)	(4,900)	(16,608)	(14,700)
Other expense, net	(1,513)	(307)	(4,534)	(1,031)
Income before income taxes	80,077	62,639	217,401	196,334
Income tax provision	(26,036)	(19,248)	(71,662)	(61,067)
Net income	$ 54,041	$ 43,391	$ 145,739	$ 135,267
Basic net income per Class A and Class B share (in dollars per share)	$ 2.15	$ 1.65	$ 5.72	$ 5.14
Shares used in basic per share calculations (in shares)	25,194	26,289	25,490	26,304
Diluted net income per Class A and Class B share (in dollars per share)	$ 2.14	$ 1.64	$ 5.69	$ 5.10
Shares used in diluted per share calculations (in shares)	25,290	26,480	25,598	26,500
Cash dividend declared per Class A and Class B share (in dollars per share)	$ 0.25	$ 0.20	$ 0.70	$ 0.56