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Stock-Based Compensation - Black-Scholes-Merton option-pricing model (Details)

v3.21.2
Stock-Based Compensation - Black-Scholes-Merton option-pricing model (Details)
9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Risk - free interest rate (percentage), minimum 0.60% 0.30% 1.60% 2.30%
Risk - free interest rate (percentage), maximum 1.10% 1.60% 2.60% 3.10%
Expected volatility (percentage), minimum 42.10% 39.90% 35.40% 34.50%
Expected volatility (percentage), maximum 43.60% 44.10% 40.90% 35.40%
Minimum        
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Expected term (years) 5 years 10 months 24 days 5 years 3 months 18 days 5 years 7 months 6 days 5 years 10 months 24 days
Maximum        
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Expected term (years) 6 years 1 month 6 days 6 years 3 months 18 days 6 years 1 month 6 days 6 years 3 months 18 days