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Stock incentive plan - Restricted stock units (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
May 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Weighted-average grant date fair value per share      
Non-cash restricted stock unit award expense recognized   $ 9,036 $ 3,318
Restricted Stock Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 2 years    
Vested and outstanding awards   21,764  
Shares      
Issued (in shares) 1,183,400 1,183,000  
Vested   (26,000)  
Forfeited and canceled (in shares)   (125,000)  
Unvested at end of period (in shares)   1,032,000  
Weighted-average grant date fair value per share      
Issued (in dollars per share)   $ 3.63  
Vested (in dollars per share)   3.63  
Forfeited and canceled (in dollars per share)   3.63  
Unvested at end of period (in dollars per share)   $ 3.63  
Non-cash restricted stock unit award expense recognized   $ 1,300  
Unrecognized compensation related to unvested restricted stock units   $ 2,600  
Expenses recognized over a weighted-average period (in years)   1 year 4 months 17 days