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Stock-Based Compensation - Summary of Restricted Stock Awards and Units (Details) - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
2010 Plan | Common Stock              
Stock-based compensation              
Maximum number of shares of common stock that may be issued       2,130,000      
Number of shares of common stock available for grant       1,017,215      
Restricted Stock Share Awards              
Shares              
Unvested at the beginning of the period (in shares)     14,701     14,701  
Vested (in shares)           (14,701)  
Unvested at the end of the period (in shares)             14,701
Weighted Average Grant Date Fair Value              
Unvested at the beginning of the period (in dollars per share)     $ 14.78     $ 14.78  
Vested (in dollars per share)           $ 14.78  
Unvested at the end of the period (in dollars per share)             $ 14.78
Weighted Average Remaining Contractual Term              
Unvested at the beginning of the period             4 days
Unvested at the end of the period             4 days
Restricted Stock Unit Awards              
Shares              
Unvested at the beginning of the period (in shares) 47,542 47,790 48,665 47,542 47,790 48,665  
Granted (in shares)       134,804 128,893 131,765  
Vested (in shares)       (136,747) (128,697) (132,640)  
Cancelled and forfeited (in shares)         (444)    
Unvested at the end of the period (in shares)       45,599 47,542 47,790 48,665
Expected to vest in the future, at the end of the period (in shares)       43,957      
Weighted Average Grant Date Fair Value              
Unvested at the beginning of the period (in dollars per share) $ 23.95 $ 20.31 $ 17.33 $ 23.95 $ 20.31 $ 17.33  
Granted (in dollars per share) $ 37.40 $ 33.60 $ 19.88 35.73 24.31 21.37  
Vested (in dollars per share)       32.45 22.93 20.27  
Cancelled and forfeited (in dollars per share)         33.60    
Unvested at the end of the period (in dollars per share)       33.28 $ 23.95 $ 20.31 $ 17.33
Expected to vest in the future, at the end of the period (in dollars per share)       $ 33.28      
Weighted Average Remaining Contractual Term              
Unvested at the beginning of the period       1 year 3 months 26 days 10 months 2 days 11 months 16 days 1 year
Granted       5 months 5 days 3 months 22 days 4 months 6 days  
Unvested at the end of the period       1 year 3 months 26 days 10 months 2 days 11 months 16 days 1 year
Expected to vest in future, at the end of the period       1 year 3 months 26 days