Acquisitions - Assets (Liabilities) Acquired (Assumed) at Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Assets (liabilities) acquired (assumed) at fair value:
Goodwill	$ 353,958		$ 348,387
Cash paid for acquisition, net of cash acquired	12,484	$ 75,448
Assets Or Stock Of Multiple Dealership Locations Acquired
Assets (liabilities) acquired (assumed) at fair value:
Inventory	6,609	29,625
Property and equipment	281	467
Other assets	55	3
Accrued liabilities	(148)	(479)
Total tangible net assets acquired	6,797	29,616
Intangible assets acquired	116
Goodwill	5,571	45,832
Purchase price	12,484	75,448
Inventory purchases financed via floor plan	(4,222)	(16,805)
Cash payment net of floor plan financing	8,262	$ 58,643
Assets Or Stock Of Multiple Dealership Locations Acquired | Membership and customer lists
Assets (liabilities) acquired (assumed) at fair value:
Intangible assets acquired	$ 116