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Stock-based Compensation - Summary of Stock Option Activity (Details)
$ / shares in Units, $ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Stock option activity    
Outstanding | shares 3,585,856  
Granted | shares 897,312  
Exercised | shares (26,911)  
Cancelled | shares (266,141)  
Outstanding | shares 4,190,116 3,585,856
Exercisable at end of period | shares 2,113,346  
Weighted average exercise price per share    
Outstanding | $ / shares $ 16.20  
Granted | $ / shares 15.67  
Exercised | $ / shares 11.30  
Cancelled | $ / shares 17.61  
Outstanding | $ / shares 16.03 $ 16.20
Exercisable at end of period | $ / shares $ 14.45  
Stock option activity, additional information    
Outstanding Weighted-Average Remaining Contractual Term 7 years 8 months 12 days 8 years
Outstanding Aggregate Intrinsic Value | $ $ 5,248 $ 11,170
Exercisable at end of period, Weighted-Average Remaining Contractual Term 6 years 7 months 6 days  
Exercisable at end of period, Aggregate Intrinsic Value | $ $ 4,285