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Stock incentive plan - Activity related to stock options (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Shares      
Granted 2,955 2,515  
Forfeited and canceled (878) (14)  
Stock options      
Shares      
Outstanding, at the beginning of the period 3,959 1,498  
Granted 1,772 2,515  
Exercised (181) (40)  
Forfeited and canceled (752) (14)  
Outstanding, at the end of the period 4,798 3,959 1,498
Vested and expected to vest 4,798    
Exercisable 2,139    
Weighted-average exercise price      
Outstanding, at the beginning of the period $ 7.46 $ 2.48  
Granted 12.19 10.40  
Exercised 2.07 2.57  
Forfeited and canceled 11.70 4.79  
Outstanding, at the end of the period 8.75 $ 7.46 $ 2.48
Vested and expected to vest 8.75    
Exercisable at the end of the period $ 5.64    
Weighted-average remaining contractual term      
Weighted-average remaining contractual term 8 years 3 months 15 days 8 years 9 months 15 days 8 years 11 months 5 days
Vested and expected to vest 8 years 3 months 15 days    
Exercisable 7 years 10 months 13 days    
Aggregate intrinsic value      
Outstanding, at the beginning of the period $ 35,984 $ 4,146  
Outstanding, at the end of the period 1,159 $ 35,984 $ 4,146
Vested and expected to vest 1,159    
Exercisable $ 928    
Weighted-average grant date fair value per share $ 7.98 $ 7.03  
Unrecognized stock-based compensation related to non-vested stock options $ 17,500    
Expenses recognized over a weighted-average period (in years) 2 years 8 months 5 days    
Term of award 10 years    
Vesting period 4 years