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Stock Option Plans - Assumptions Used to Value Option Grants (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected life in years   5 years
Interest rate, minimum 1.52% 2.43%
Interest rate, maximum 2.39% 3.02%
Volatility, minimum 0.358% 0.287%
Volatility, maximum 0.375% 0.316%
Dividend yield 2.0 - 2.2% 1.8 - 2.75%
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