Goodwill and Intangible Assets, Net (Intangible Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Indefinite-lived Intangible Assets by Segment [Line Items]
Franchise rights - indefinite-lived	$ 580.1	$ 572.2
AN Reportable Segment, Domestic [Member]
Indefinite-lived Intangible Assets by Segment [Line Items]
Franchise rights - indefinite-lived	160.5
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	$ 310.7