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Stock-Based Compensation - Schedule of Restricted Stock Activity (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]      
Ending Balance $ 1,693.9 $ 1,427.6 $ 982.6
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock issued during period, shares, restricted stock award 214,893    
Stock compensation expense $ 5.6 $ 3.3 3.9
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Restricted stock, beginning of year, number of shares 292,506    
Granted, number of shares 214,893    
Vested, number of shares (83,736)    
Forfeited, number of shares (43,079)    
Canceled, number of shares 0    
Restricted stock, end of year, number of shares 380,584 292,506  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]      
Restricted stock, beginning of year, weighted-average measurement date fair value (in dollars per share) $ 33.24    
Granted, weighted average grant date fair value (in dollars per share) 40.97    
Vested, weighted average grant date fair value (in dollars per share) 32.42    
Forfeited, weighted average grant date fair value (in dollars per share) 36.64    
Canceled, weighted average grant date fair value (in dollars per share) 0.00    
Restricted stock, end of year, weighted-average measurement date fair value (in dollars per share) $ 37.46 $ 33.24  
Tax benefits from stock-based compensation $ 0.2 $ 0.6 1.5
Unrecognized compensation cost related to nonvested restricted stock awards $ 7.9    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 10 months 28 days    
Common Stock      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]      
Ending Balance $ 866.7 $ 687.0 $ 296.1
Upon Achievement of Defined Total Return to Shareholder Goals | Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock issued during period, shares, restricted stock award 29,732