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Stock Plans - Summary of Stock Option Activity (Details)
 6 Months Ended
Jun. 30, 2018
$ / shares
shares
Shares  
Outstanding, beginning balance (in shares) | shares 955,446
Exercised (in shares) | shares (118,957)
Forfeited (in shares) | shares 3,358
Outstanding, ending balance (in shares) | shares 833,131
Exercisable (in shares) | shares 719,299
Weighted Average Exercise Price (dollars per share)  
Outstanding, beginning balance (dollars per share) | $ / shares $ 42.75
Exercised (dollars per share) | $ / shares 36.09
Forfeited (dollars per share) | $ / shares 44.50
Outstanding, ending balance (dollars per share) | $ / shares 43.72
Exercisable (dollars per share) | $ / shares $ 43.59