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Earnings Per Common Share - Calculation of diluted EPS under Treasury method (Details) - $ / shares
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Earnings Per Share    
Number of shares 62,235 62,235
Range of exercise prices, low end of range (in dollars per share) $ 87.30 $ 87.30
Range of exercise prices, high end of range (in dollars per share) $ 91.35 $ 91.35