Acquisitions - Purchase Price (Details) - Assets Or Stock Of Multiple Dealership Locations Acquired - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Acquisitions
Purchase Price	$ 78,606	$ 126,689	$ 10,615
Purchase price holdback included in other long-term liabilities		(1,500)
Inventory purchases financed via floor plan	(28,942)	(59,794)	(4,029)
Cash payment net of holdback and floor plan financing	49,664	65,395	6,586
Goodwill for tax purposes	$ 40,600	$ 22,700	$ 4,400