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Employee Benefits and Share-Based Compensation - Assumptions Used to Value ESPP Shares Granted (Details) - ESPP shares available for future issuance - 1997 Plan
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected volatility (minimum) 27.40% 30.40%
Expected volatility (maximum) 53.50% 39.90%
Risk-free interest rate (minimum) 0.10% 1.40%
Risk-free interest rate (maximum) 2.60% 2.70%
Dividend yield, % 0.00% 0.00%
Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected life 6 months 6 months
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected life 2 years 2 years