Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
New Vehicle [Member]
Revenues:
Revenues	$ 1,209,037	$ 1,149,512	$ 2,305,092	$ 2,157,328
Cost of sales:
Cost of sales	1,136,175	1,080,170	2,165,464	2,026,212
Used Retail Vehicle [Member]
Revenues:
Revenues	553,647	488,801	1,086,373	951,732
Cost of sales:
Cost of sales	486,422	426,108	954,871	829,597
Used Wholesale Vehicle [Member]
Revenues:
Revenues	66,714	66,796	131,860	129,004
Cost of sales:
Cost of sales	65,228	65,390	128,544	125,437
Finance and Insurance [Member]
Revenues:
Revenues	81,043	72,463	158,681	137,067
Service, Body and Parts [Member]
Revenues:
Revenues	202,265	182,695	398,940	356,170
Cost of sales:
Cost of sales	103,666	91,946	204,222	180,982
Fleet and Other [Member]
Revenues:
Revenues	20,633	36,680	35,254	54,824
Cost of sales:
Cost of sales	19,812	35,684	33,881	52,873
Revenues	2,133,339	1,996,947	4,116,200	3,786,125
Cost of sales	1,811,303	1,699,298	3,486,982	3,215,101
Gross profit	322,036	297,649	629,218	571,024
Asset impairments	3,498	6,130	6,996	10,260
Selling, general and administrative	215,526	195,610	434,632	387,228
Depreciation and amortization	12,503	10,287	24,166	20,013
Operating income	90,509	85,622	163,424	153,523
Floor plan interest expense	(6,209)	(4,655)	(12,118)	(9,304)
Other interest expense, net	(5,502)	(4,972)	(10,961)	(9,800)
Other expense, net	(1,495)	(356)	(3,021)	(724)
Income before income taxes	77,303	75,639	137,324	133,695
Income tax provision	(25,875)	(24,416)	(45,626)	(41,819)
Net income	$ 51,428	$ 51,223	$ 91,698	$ 91,876
Basic net income per share (in dollars per share)	$ 2.02	$ 1.95	$ 3.58	$ 3.49
Shares used in basic per share calculations (in shares)	25,462	26,332	25,639	26,310
Diluted net income per share (in dollars per share)	$ 2.01	$ 1.93	$ 3.56	$ 3.47
Shares used in diluted per share calculations (in shares)	25,534	26,496	25,754	26,509