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Stock Option Plans - Summary of Option Activity (Details)
 6 Months Ended
Jun. 30, 2017
USD ($)
$ / shares
shares
Number of Shares  
Outstanding at beginning of period (in shares) | shares 833,630
Granted (in shares) | shares 232,500
Exercised (in shares) | shares (73,060)
Expired/Forfeited (in shares) | shares (3,750)
Outstanding at end of period (in shares) | shares 989,320
Exercisable at end of period (in shares) | shares 506,945
Weighted Average Exercise Price per share  
Outstanding at beginning of period (in usd per share) | $ / shares $ 49.92
Granted (in usd per share) | $ / shares 59.41
Exercised (in usd per share) | $ / shares 46.63
Expired/Forfeited (in usd per share) | $ / shares 53.73
Outstanding at end of period (in usd per share) | $ / shares 52.38
Exercisable at end of period (in usd per share) | $ / shares $ 48.69
Aggregate Intrinsic Value  
Outstanding at beginning of period | $ $ 13,913,891
Granted | $ 0
Exercised | $ 1,121,042
Expired/Forfeited | $
Outstanding at end of period | $ 5,874,639
Exercisable at end of period | $ $ 4,749,306