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Stock-Based Compensation Plans (Details 4) - Performance awards - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share Based Compensation Arrangement By Share Based Payment Award Maximum Measurement Period 3 years    
Performance awards      
Outstanding at the beginning of the period (in shares) 253,432 267,129 251,770
Granted (in shares) 77,182 80,512 100,965
Increase (decrease) in awards based on performance (in shares) 18,112 (16,760) 5,990
Converted to restricted stock (in shares) (96,317) (73,072) (88,477)
Forfeited (in shares) (10,488) (4,377) (3,119)
Outstanding at the ending of the period (in shares) 241,921 253,432 267,129
Weighted average grant date fair value per award      
Outstanding at the beginning of the period (in dollars per share) $ 176.27 $ 157.21 $ 155.25
Granted (in dollars per share) 238.03 200.75 155.31
Change in awards based on performance (in dollars per share) 177.26 142.03 148.79
Converted to restricted stock (in dollars per share) 177.26 142.03 148.79
Forfeited (in dollars per share) 188.52 166.44 160.33
Outstanding at the ending of the period (in dollars per share) $ 195.13 $ 176.27 $ 157.21
Grants in period based on shareholder metrics (in shares) 38,823 47,502 62,043
Grants in period based on financial metrics (in shares) 38,359 33,010 38,922