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Stock-Based Compensation Plans (Details 6) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Restricted stock      
Restricted stock shares      
Outstanding at the beginning of the period (in shares) 148,326 160,411 133,633
Granted (in shares) 69,228 79,430 98,713
Vested (in shares) (79,931) (89,289) (67,832)
Forfeited (in shares) (5,899) (2,226) (4,103)
Outstanding at the ending of the period (in shares) 131,724 148,326 160,411
Restricted stock shares weighted average grant date fair value per share      
Outstanding at the beginning of the period (in dollars per share) $ 181.29 $ 166.33 $ 172.33
Granted (in dollars per share) 221.08 196.43 161.58
Vested (in dollars per share) 178.41 168.06 171.22
Forfeited (in dollars per share) 196.22 174.45 166.40
Outstanding at the ending of the period (in dollars per share) $ 203.28 $ 181.29 $ 166.33
Restricted stock converted from performance shares      
Restricted stock shares      
Outstanding at the beginning of the period (in shares) 163,111 209,238 233,928
Granted (in shares) 96,317 73,072 88,297
Vested (in shares) (111,325) (119,064) (112,230)
Forfeited (in shares) (1,784) (135) (757)
Outstanding at the ending of the period (in shares) 146,319 163,111 209,238