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Description of Business and Summary of Significant Accounting Policies (Revenue Recognition, Advertising) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Advertising [Abstract]        
Advertising expense, net $ 188.5 $ 164.9 $ 166.4  
Manufacturer advertising reimbursements 56.4 47.1 42.4  
Chargeback Reserves [Member]        
Revenue Recognition [Abstract]        
Chargeback liabilities $ 97.3 $ 84.9 $ 67.6 $ 56.0