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Stock Based Compensation - Summary of Restricted Stock Awards and Units (Details) - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
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       918,830      
Restricted stock units              
Shares              
Unvested at the beginning of the period (in shares) 45,599 47,542 47,790 45,599 47,542 47,790  
Granted (in shares)       47,360 134,804 128,893  
Vested (in shares)       (56,863) (136,747) (128,697)  
Cancelled and forfeited (in shares)       (420)   (444)  
Unvested at the end of the period (in shares)       35,676 45,599 47,542 47,790
Expected to vest in the future, at the end of the period (in shares)       34,392      
Weighted Average Grant Date Fair Value              
Unvested at the beginning of the period (in dollars per share) $ 33.28 $ 23.95 $ 20.31 $ 33.28 $ 23.95 $ 20.31  
Granted (in dollars per share) $ 36.48 $ 37.40 $ 33.60 36.48 35.73 24.31  
Vested (in dollars per share)       22.05 32.45 22.93  
Cancelled and forfeited (in dollars per share)       36.48   33.60  
Unvested at the end of the period (in dollars per share)       36.49 $ 33.28 $ 23.95 $ 20.31
Expected to vest in the future, at the end of the period (in dollars per share)       $ 36.49      
Weighted Average Remaining Contractual Term              
Unvested at the beginning of the period       1 year 4 months 24 days 1 year 3 months 25 days 10 months 2 days 11 months 15 days
Granted       9 months 3 days 5 months 4 days 3 months 21 days  
Unvested at the end of the period       1 year 4 months 24 days 1 year 3 months 25 days 10 months 2 days 11 months 15 days
Expected to vest in future, at the end of the period       1 year 4 months 24 days