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Stock-Based Compensation Plans Stock-Based Compensation Plans (Details 4) - $ / shares
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Restricted stock    
Restricted stock shares    
Equity instruments outstanding at the beginning of the period (in shares) 160,411  
Equity instruments granted (in shares) 77,287 98,085
Vested (in shares) (77,244)  
Forfeited (in shares) (1,438)  
Equity instruments outstanding at the end of the period (in shares) 159,016  
Restricted stock shares weighted average grant date fair value per share    
Equity instruments outstanding at the beginning of the period (in dollars per share) $ 166.33  
Grant date fair value per share (in dollars per share) 195.96  
Vested (in dollars per share) 166.52  
Forfeited (in dollars per share) 174.19  
Equity instruments outstanding at the end of the period (in dollars per share) $ 180.57  
Restricted Stock Converted From Performance Shares    
Restricted stock shares    
Equity instruments outstanding at the beginning of the period (in shares) 209,238  
Equity instruments granted (in shares) 73,072 88,297
Vested (in shares) (110,366)  
Forfeited (in shares) 0  
Equity instruments outstanding at the end of the period (in shares) 171,944