Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Total revenues	$ 10,086,510	$ 8,678,157	$ 7,864,252
Cost of sales:
Total cost of sales	8,570,424	7,376,842	6,688,618
Gross profit	1,516,086	1,301,315	1,175,634
Asset impairments	0	13,992	20,124
Selling, general and administrative	1,049,378	899,590	811,175
Depreciation and amortization	57,722	49,369	41,600
Operating income	408,986	338,364	302,735
Floor plan interest expense	(39,336)	(25,531)	(19,534)
Other interest expense	(34,776)	(23,207)	(19,491)
Other (expense) income, net	12,195	(6,103)	(1,006)
Income before income taxes	347,069	283,523	262,704
Income tax provision	(101,852)	(86,465)	(79,705)
Net income	$ 245,217	$ 197,058	$ 182,999
Basic net income per share (in dollars per share)	$ 9.78	$ 7.76	$ 6.96
Shares used in basic per share calculations (in shares)	25,065	25,409	26,290
Diluted net income per share (in dollars per share)	$ 9.75	$ 7.72	$ 6.91
Shares used in diluted per share calculations (in shares)	25,145	25,521	26,490
Cash dividend paid per Class A and Class B share (in dollars per share)	$ 1.06	$ 0.95	$ 0.76
New vehicle
Revenues:
Total revenues	$ 5,763,587	$ 4,938,436	$ 4,552,301
Cost of sales:
Total cost of sales	5,423,744	4,649,024	4,271,931
Used vehicle retail
Revenues:
Total revenues	2,544,379	2,226,951	1,927,016
Cost of sales:
Total cost of sales	2,257,544	1,963,267	1,685,767
Used vehicle wholesale
Revenues:
Total revenues	277,844	276,616	261,530
Cost of sales:
Total cost of sales	273,058	272,303	257,073
Finance and insurance
Revenues:
Total revenues	385,863	330,922	283,018
Service, body and parts
Revenues:
Total revenues	1,015,773	844,505	738,990
Cost of sales:
Total cost of sales	522,649	434,222	375,069
Fleet and other
Revenues:
Total revenues	99,064	60,727	101,397
Cost of sales:
Total cost of sales	$ 93,429	$ 58,026	$ 98,778