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Earnings Per Common Share - Calculation of diluted EPS under Treasury method (Details) - $ / shares
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Earnings Per Share        
Number of shares 57,169 141,116 60,546 136,844
Range of exercise prices, low end of range (in dollars per share) $ 87.30 $ 52.28 $ 87.30 $ 61.42
Range of exercise prices, high end of range (in dollars per share) $ 91.35 $ 91.35 $ 91.35 $ 91.35