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Stock Option Plans - Summary of Option Activity (Details)
 9 Months Ended
Sep. 30, 2018
USD ($)
$ / shares
shares
Number of Shares  
Outstanding at beginning of period (in shares) | shares 913,320
Granted (in shares) | shares 245,000
Exercised (in shares) | shares (29,151)
Expired/Forfeited (in shares) | shares (5,000)
Outstanding at end of period (in shares) | shares 1,124,169
Exercisable at end of period (in shares) | shares 610,919
Weighted Average Exercise Price per share  
Outstanding at beginning of period (in usd per share) | $ / shares $ 52.80
Granted (in usd per share) | $ / shares 49.46
Exercised (in usd per share) | $ / shares 44.64
Expired/Forfeited (in usd per share) | $ / shares 54.78
Outstanding at end of period (in usd per share) | $ / shares 52.28
Exercisable at end of period (in usd per share) | $ / shares $ 50.72
Aggregate Intrinsic Value  
Outstanding at beginning of period | $ $ 8,172,970
Granted | $ 1,602,300
Exercised | $ 415,119
Expired/Forfeited | $
Outstanding at end of period | $ 5,341,971
Exercisable at end of period | $ $ 3,691,508