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 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
2010 Plan | Common Stock    
Stock-based compensation    
Maximum number of shares of common stock that may be issued 2,130,000  
Restricted stock units    
Shares    
Unvested at the beginning of the period (in shares) 45,599  
Granted (in shares) 47,360  
Vested (in shares) (35,113)  
Unvested at the end of the period (in shares) 57,846 45,599
Expected to vest in the future, at the end of the period (in shares) 57,846  
Weighted Average Grant Date Fair Value    
Unvested at the beginning of the period (in dollars per share) $ 33.28  
Granted (in dollars per share) 36.48  
Vested (in dollars per share) 32.64  
Unvested at the end of the period (in dollars per share) 36.46 $ 33.28
Expected to vest in the future, at the end of the period (in dollars per share) $ 36.46  
Weighted Average Remaining Contractual Term    
Unvested at the beginning of the period 1 year 5 months 19 days 1 year 3 months 26 days
Granted 1 year 1 month 21 days  
Unvested at the end of the period 1 year 5 months 19 days 1 year 3 months 26 days
Expected to vest in future, at the end of the period 1 year 5 months 19 days