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Stock Compensation Plans - Performance Shares (Details) - Performance Shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility of common stock 19.60% 19.40% 20.80%
Average correlation coefficient of peer companies 51.00% 50.00% 54.00%
Risk-free spot rate 1.20% 2.40% 2.40%
Term 3 years 3 years 3 years
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Average volatility of peer companies 18.00% 16.00% 17.00%
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Average volatility of peer companies 31.00% 27.00% 25.00%