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Stock-Based Compensation Plans (Details 6) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted stock      
Restricted stock shares      
Outstanding at the beginning of the period (in shares) 136,705 147,884 179,307
Granted (in shares) 73,342 81,400 61,953
Vested (in shares) (73,683) (88,712) (91,847)
Forfeited (in shares) (2,731) (3,867) (1,529)
Outstanding at the ending of the period (in shares) 133,633 136,705 147,884
Restricted stock shares weighted average grant date fair value per share      
Outstanding at the beginning of the period (in dollars per share) $ 158.51 $ 146.21 $ 129.06
Granted (in dollars per share) 179.58 162.38 173.04
Vested (in dollars per share) 153.86 141.38 130.75
Forfeited (in dollars per share) 173.42 162.43 151.86
Outstanding at the ending of the period (in dollars per share) $ 172.33 $ 158.51 $ 146.21
Restricted stock converted from performance shares      
Restricted stock shares      
Outstanding at the beginning of the period (in shares) 176,698 98,347 10,933
Granted (in shares) 128,482 115,618 95,826
Vested (in shares) (70,595) (36,872) (8,412)
Forfeited (in shares) (657) (395) 0
Outstanding at the ending of the period (in shares) 233,928 176,698 98,347