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Stock incentive plans - Activity related to stock options (Details) - Stock options - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Shares      
Outstanding, at the beginning of the period 3,959    
Granted 1,663 2,400  
Exercised (142)    
Forfeited and canceled (522)    
Outstanding, at the end of the period 4,958   3,959
Vested and expected to vest 4,958    
Exercisable 1,877    
Weighted-average exercise price      
Outstanding, at the beginning of the period $ 7.46    
Granted 12.77    
Exercised 1.99    
Forfeited and canceled 11.91    
Outstanding, at the end of the period 8.93   $ 7.46
Vested and expected to vest 8.93    
Exercisable at the end of the period $ 5.37    
Weighted-average remaining contractual term      
Weighted-average remaining contractual term 8 years 5 months 12 days   8 years 9 months 15 days
Vested and expected to vest 8 years 5 months 12 days    
Exercisable 7 years 10 months 24 days    
Aggregate intrinsic value      
Outstanding, at the beginning of the period $ 35,984    
Outstanding, at the end of the period 1,356   $ 35,984
Vested and expected to vest 1,356    
Exercisable $ 968    
Weighted-average grant date fair value per share $ 8.36 $ 9.95  
Unrecognized stock-based compensation related to non-vested stock options $ 20,400    
Expenses recognized over a weighted-average period (in years) 2 years 10 months 24 days    
Term of award 10 years    
Vesting period 4 years