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Stock-Based Compensation Expense - PSUs Fair Value Calculation Assumptions (Details) - Stock-Settled Performance-Based Restricted Stock Units
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate, minimum 0.10% 1.10%
Risk-free interest rate, maximum 0.30% 1.30%
Expected price volatility, minimum 43.70% 40.20%
Expected price volatility, maximum 49.30% 56.10%
Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected option term / Performance Period 1 year 9 months 18 days 9 months 18 days
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected option term / Performance Period 2 years 9 months 18 days 2 years 9 months 18 days