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Employee Benefits and Share-Based Compensation - Assumptions Used to Value ESPP Shares Granted (Details) - ESPP - 1997 Plan
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility (minimum) 30.40% 28.20% 28.10%
Expected volatility (maximum) 53.50% 39.90% 33.80%
Risk-free interest rate (minimum) 0.10% 1.30% 0.80%
Risk-free interest rate (maximum) 2.70% 2.70% 2.70%
Dividend yield 0.00% 0.00% 0.00%
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected life 6 months 6 months 6 months
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected life 2 years 2 years 2 years