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Stock-based Compensation - Stock Options (Details) - 2009 plan - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Number of Shares    
Outstanding at the beginning of the period (in shares) 7,453,076  
Granted (in shares) 1,469,422  
Exercised (in shares) (61,158)  
Forfeited (43,397)  
Outstanding at the end of the period (in shares) 8,817,943 7,453,076
Vested and expected to vest (in shares) 8,817,943  
Options exercisable (in shares) 4,441,194  
Weighted Average Exercise Price    
Outstanding at the beginning of the period (in dollars per share) $ 7.46  
Granted (in dollars per share) 3.85  
Exercised (in dollars per share) 2.71  
Forfeited (in dollars per share) 6.36  
Outstanding at the end of the period (in dollars per share) 6.90 $ 7.46
Vested and expected to vest (in dollars per share ) 6.90  
Options exercisable (in dollars per shares) $ 7.14  
Weighted Average Remaining Contractual Term    
Weighted average period 7 years 10 months 24 days 7 years 8 months 12 days
Vested and expected to vest 7 years 10 months 24 days  
Options exercisable 6 years 10 months 24 days  
Aggregate Intrinsic Value    
Outstanding at the beginning of the period (in dollars) $ 1,313  
Outstanding at the end of the period (in dollars) 30,866 $ 1,313
Vested and expected to vest (in dollars) 30,866  
Options exercisable (in dollars) $ 15,609