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Inventory and Vehicle Floorplan Payable (Components of Inventory) (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Inventory [Line Items]    
Inventory $ 3,612.0 $ 2,899.0
New Vehicle [Member]    
Inventory [Line Items]    
Inventory 2,888.1 2,294.3
Used Vehicle [Member]    
Inventory [Line Items]    
Inventory 539.7 437.6
Parts and Service [Member]    
Inventory [Line Items]    
Inventory $ 184.2 $ 167.1