Quarterly report pursuant to Section 13 or 15(d)

Employee Stock Plans - Summary of Restricted Stock Awards and Units (Details)

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Employee Stock Plans - Summary of Restricted Stock Awards and Units (Details) - $ / shares
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
2010 Plan      
Stock-based compensation      
Maximum number of shares of common stock that may be issued 2,130,000 2,130,000  
Performance Share Unit Awards | 2010 Plan      
Weighted Average Grant Date Fair Value      
Granted (in dollars per share) $ 37.40    
Restricted Stock Unit Awards      
Shares      
Unvested at the beginning of the period (in shares)   47,542  
Granted (in shares)   134,585  
Vested (in shares)   (96,034)  
Unvested at the end of the period (in shares) 86,093 86,093 47,542
Expected to vest in the future, at the end of the period (in shares) 86,093 86,093  
Weighted Average Grant Date Fair Value      
Unvested at the beginning of the period (in dollars per share)   $ 23.95  
Granted (in dollars per share)   35.71  
Vested (in dollars per share)   30.54  
Cancelled and forfeited (in dollars per share)   0.00  
Unvested at the end of the period (in dollars per share) $ 34.98 34.98 $ 23.95
Expected to vest in the future, at the end of the period (in dollars per share) $ 34.98 $ 34.98  
Weighted Average Remaining Contractual Term      
Unvested at the beginning of the period   11 months 16 days 10 months 2 days
Granted   6 months 26 days  
Unvested at the end of the period   11 months 16 days 10 months 2 days
Expected to vest in future, at the end of the period   11 months 16 days