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Stock Based Compensation - Restricted stock units (Details) - Restricted stock units
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2021
USD ($)
$ / shares
shares
Mar. 31, 2021
USD ($)
$ / shares
shares
Stock-based compensation    
Beginning balance | shares   358,825
Granted | shares   247,050
Vested | shares   (117,808)
Forfeited | shares   (9,071)
Ending balance | shares 478,996 478,996
Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares   $ 33.04
Weighted Average Grant Date Fair Value, Granted | $ / shares   43.11
Weighted Average Grant Date Fair Value, Vested | $ / shares   31.75
Weighted Average Grant Date Fair Value, Forfeited | $ / shares   32.36
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares $ 38.56 $ 38.56
Compensation expense    
Unamortized compensation expense related to unvested restricted stock units | $ $ 17,300 $ 17,300
Period to recognize unamortized compensation expense 2 years 7 months 6 days