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Stock Option Plans - Assumptions Used to Value Option Grants (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected life in years 6 years  
Interest rate, minimum 0.30% 1.52%
Interest rate, maximum 1.63% 2.39%
Volatility, minimum 0.358% 0.358%
Volatility, maximum 0.384% 0.375%
Dividend yield 2.0 - 2.2% 2.0 - 2.2%
Minimum Range    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected life in years   5 years
Maximum Range    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected life in years   6 years