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Stock incentive plan - Activity related to stock options (Details) - Stock options - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Shares      
Outstanding, at the beginning of the period 4,798    
Granted 1,939 1,000  
Exercised (104)    
Forfeited and canceled (214)    
Outstanding, at the end of the period 6,419   4,798
Vested and expected to vest 6,419    
Exercisable 2,439    
Weighted-average exercise price      
Outstanding, at the beginning of the period $ 8.75    
Granted 3.09    
Exercised 2.34    
Forfeited and canceled 6.12    
Outstanding, at the end of the period 7.23   $ 8.75
Vested and expected to vest 7.23    
Exercisable at the end of the period $ 6.96    
Weighted-average remaining contractual term      
Weighted-average remaining contractual term 8 years 5 months 23 days   8 years 3 months 15 days
Vested and expected to vest 8 years 5 months 23 days    
Exercisable 7 years 5 months 12 days    
Aggregate intrinsic value      
Outstanding, at the beginning of the period $ 1,159    
Outstanding, at the end of the period 102   $ 1,159
Vested and expected to vest 102    
Exercisable $ 100    
Weighted-average grant date fair value per share $ 2.04 $ 12.60  
Unrecognized stock-based compensation related to non-vested stock options $ 19,000    
Expenses recognized over a weighted-average period (in years) 2 years 9 months    
Term of award 10 years    
Vesting period 4 years