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Stock-Based Compensation Plans Stock-Based Compensation Plans (Details 4) - $ / shares
6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Restricted stock    
Restricted stock shares    
Equity instruments outstanding at the beginning of the period (in shares) 148,326  
Equity instruments granted (in shares) 68,209 77,287
Vested (in shares) (76,893)  
Forfeited (in shares) (3,456)  
Equity instruments outstanding at the end of the period (in shares) 136,186  
Restricted stock shares weighted average grant date fair value per share    
Equity instruments outstanding at the beginning of the period (in dollars per share) $ 181.29  
Grant date fair value per share (in dollars per share) 222.12  
Vested (in dollars per share) 178.57  
Forfeited (in dollars per share) 191.73  
Equity instruments outstanding at the end of the period (in dollars per share) $ 203.00  
Restricted Stock Converted From Performance Shares    
Restricted stock shares    
Equity instruments outstanding at the beginning of the period (in shares) 163,111  
Equity instruments granted (in shares) 96,317 73,072
Vested (in shares) (109,758)  
Forfeited (in shares) (594)  
Equity instruments outstanding at the end of the period (in shares) 149,076