Business And Credit Concentrations (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Risks and Uncertainties [Abstract]
Percentage of new vehicle sales from core brands (percent)	93.00%
Manufacturer receivables	$ 222.9	$ 234.9
Percentage of revenue from stores located in Florida, Texas and California	62.00%
Unrecovered hurricane-related losses	$ 3.0