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Share-Based Compensation - Assumptions For Stock Options (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-Based Compensation        
Share-based compensation expense before tax   $ 45.4 $ (26.4) $ 73.5
Stock Options        
Share-Based Compensation        
Granted (in shares)   2,081,047    
Total grant date fair value of employee stock options that vested   $ 37.4 33.9 13.1
Share-based compensation expense before tax   $ 70.5 $ 58.5 $ 43.0
Expected volatility maximum period   5 years    
Method and assumptions on valuation of stock options        
Expected term of awards (in years)   5 years 9 months 18 days 6 years 3 months 18 days 6 years 1 month 6 days
Expected volatility (as a percent)   33.80% 36.20% 35.70%
Risk-free interest rate (as a percent)   2.40% 2.70% 2.20%
Expected dividend yield (as a percent)   0.00% 0.00% 0.00%
Stock Options | 2015 Plan | Minimum        
Share-Based Compensation        
Share-based awards, vesting period 1 year      
Stock Options | 2015 Plan | Maximum        
Share-Based Compensation        
Share-based awards, vesting period 3 years