Segments Information - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segments Information
Revenue	$ 1,312,727	$ 1,235,602	$ 3,819,469	$ 3,396,263
Intersegment Eliminations
Segments Information
Revenue	(40,378)	(36,251)	(115,997)	(103,894)
Consumer Services and Plans
Segments Information
Revenue	52,044	46,169	158,600	144,518
Consumer Services and Plans | Intersegment Eliminations
Segments Information
Revenue	(182)	(173)	(1,909)	(1,414)
New vehicles
Segments Information
Revenue	697,317	713,362	2,084,346	1,977,472
New vehicles | Intersegment Eliminations
Segments Information
Revenue	(1,946)	(1,604)	(6,018)	(5,172)
Used vehicles
Segments Information
Revenue	197,757	187,463	580,494	528,897
Used vehicles | Intersegment Eliminations
Segments Information
Revenue	(798)	(1,084)	(2,322)	(2,427)
Dealership parts, services and other
Segments Information
Revenue	71,607	66,847	210,024	185,586
Finance and insurance, net
Segments Information
Revenue	109,459	100,858	325,368	267,207
Finance and insurance, net | Intersegment Eliminations
Segments Information
Revenue	(3,018)	(2,277)	(8,920)	(6,015)
Retail
Segments Information
Revenue	184,543	120,903	460,637	292,583
Retail | Intersegment Eliminations
Segments Information
Revenue	(34,434)	(31,113)	(96,828)	(88,866)
Consumer Services and Plans
Segments Information
Revenue	52,044	46,169	158,600	144,518
Consumer Services and Plans | Operating Segments
Segments Information
Revenue	52,226	46,342	160,509	145,932
Consumer Services and Plans | Consumer Services and Plans | Operating Segments
Segments Information
Revenue	52,226	46,342	160,509	145,932
Dealership | Operating Segments
Segments Information
Revenue	1,081,902	1,073,495	3,217,492	2,972,776
Dealership | New vehicles
Segments Information
Revenue	697,317	713,362	2,084,346	1,977,472
Dealership | New vehicles | Operating Segments
Segments Information
Revenue	699,263	714,966	2,090,364	1,982,644
Dealership | Used vehicles
Segments Information
Revenue	197,757	187,463	580,494	528,897
Dealership | Used vehicles | Operating Segments
Segments Information
Revenue	198,555	188,547	582,816	531,324
Dealership | Dealership parts, services and other
Segments Information
Revenue	71,607	66,847	210,024	185,586
Dealership | Dealership parts, services and other | Operating Segments
Segments Information
Revenue	71,607	66,847	210,024	185,586
Dealership | Finance and insurance, net
Segments Information
Revenue	109,459	100,858	325,368	267,207
Dealership | Finance and insurance, net | Operating Segments
Segments Information
Revenue	112,477	103,135	334,288	273,222
Retail
Segments Information
Revenue	184,543	120,903	460,637	292,583
Retail | Operating Segments
Segments Information
Revenue	218,977	152,016	557,465	381,449
Retail | Retail | Operating Segments
Segments Information
Revenue	$ 218,977	$ 152,016	$ 557,465	$ 381,449