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Stock Plans - Summary of Stock Option Activity (Details)
 3 Months Ended
Mar. 31, 2018
$ / shares
shares
Shares  
Outstanding, beginning balance (in shares) | shares 955,446
Exercised (in shares) | shares (24,058)
Outstanding, ending balance (in shares) | shares 931,388
Exercisable (in shares) | shares 727,302
Weighted Average Exercise Price (dollars per share)  
Outstanding, beginning balance (dollars per share) | $ / shares $ 42.75
Exercised (dollars per share) | $ / shares 35.23
Outstanding, ending balance (dollars per share) | $ / shares 42.95
Exercisable (dollars per share) | $ / shares $ 42.51