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Stock-Based Compensation Plans Stock-Based Compensation Plans (Details 4) - $ / shares
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Restricted stock    
Restricted stock shares    
Equity instruments outstanding at the beginning of the period (in shares) 131,724  
Equity instruments granted (in shares) 96,834 69,109
Vested (in shares) (68,702)  
Forfeited (in shares) (3,077)  
Equity instruments outstanding at the end of the period (in shares) 156,779  
Restricted stock shares weighted average grant date fair value per share    
Equity instruments outstanding at the beginning of the period (in dollars per share) $ 203.28  
Grant date fair value per share (in dollars per share) 177.49  
Vested (in dollars per share) 192.07  
Forfeited (in dollars per share) 193.47  
Equity instruments outstanding at the end of the period (in dollars per share) $ 192.45  
Restricted Stock Converted From Performance Shares    
Restricted stock shares    
Equity instruments outstanding at the beginning of the period (in shares) 146,319  
Equity instruments granted (in shares) 0 96,317
Vested (in shares) (71,692)  
Forfeited (in shares) 0  
Equity instruments outstanding at the end of the period (in shares) 74,627