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Stock-Based Compensation - Fair Value Assumptions (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2019
Sep. 30, 2018
Stock-based compensation      
Unamortized stock-based compensation expense not yet recognized $ 54.4 $ 54.4  
RSU      
Stock-based compensation      
Unamortized stock-based compensation expense not yet recognized 25.5 $ 25.5  
Options remaining vesting period   4 years  
Weighted average period of recognition   1 year 7 months 6 days  
Stock options      
Stock-based compensation      
Unamortized stock-based compensation expense not yet recognized $ 28.9 $ 28.9  
Options remaining vesting period   4 years  
Weighted average period of recognition   2 years 1 month 6 days  
Stock-based awards - weighted average assumptions used to estimate fair value of options granted      
Risk-free interest rate (as a percent) 1.61% 2.17% 2.67%
Expected dividend yield (as a percent) 0.00% 0.00% 0.00%
Expected volatility rate (as a percent) 47.00% 46.80% 44.90%
Expected term 6 years 3 days 6 years 3 days 6 years 1 month 6 days