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Stock-Based Compensation - Stock Option Plans (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Options granted (in shares) 0 0 0  
Intrinsic value of options exercised $ 17.9 $ 11.5 $ 7.5  
Intrinsic value of options outstanding 196.6      
Excess tax benefit 30.1 20.5 7.7  
Employee stock options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense     $ 1.6  
Employee stock options and performance stock options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense $ 0.0 $ 0.6    
2015 Stock incentive plan | Employee stock options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Maximum term of stock options granted 10 years      
2017 Performance metric, targeted free cash flow performance metric | PSOs        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
PSOs vested (in shares) 77,000 40,000    
Percent of achievement of award target performance metric 95.00% 115.00%    
Class A common stock | 2015 Stock incentive plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Aggregate number of shares reserved for stock incentive plan 4,026,493     2,000,000