Goodwill and Intangible Assets, Net (Intangible Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Indefinite-lived Intangible Assets by Segment [Line Items]
Franchise rights - indefinite-lived	$ 572.2	$ 589.4
AN Reportable Segment, Domestic [Member]
Indefinite-lived Intangible Assets by Segment [Line Items]
Franchise rights - indefinite-lived	168.0
AN Reportable Segment, Import [Member]
Indefinite-lived Intangible Assets by Segment [Line Items]
Franchise rights - indefinite-lived	108.9
AN Reportable Segment, Premium Luxury [Member]
Indefinite-lived Intangible Assets by Segment [Line Items]
Franchise rights - indefinite-lived	$ 295.3