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Stock Plans - Summary of Restricted Stock and Unit Activity (Details) - Restricted Stock Activity
6 Months Ended
Jun. 30, 2018
$ / shares
shares
Shares  
Unvested, beginning balance (in shares) | shares 227,161
Granted (in shares) | shares 65,133
Vested (in shares) | shares (81,031)
Forfeited (in shares) | shares (3,555)
Unvested, ending balance (in shares) | shares 207,708
Weighted Average Exercise Price (dollars per share)  
Unvested, beginning balance (dollars per share) | $ / shares $ 57.50
Granted (dollars per share) | $ / shares 85.57
Vested (dollars per share) | $ / shares 58.32
Forfeited (dollars per share) | $ / shares 57.76
Unvested, ending Balance (dollars per share) | $ / shares $ 66.89