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Stock incentive plans - Restricted stock units (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 9 Months Ended
May 31, 2019
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Weighted-average grant date fair value per share          
Non-cash restricted stock unit award expense recognized   $ 2,641 $ 720 $ 6,966 $ 1,984
Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period 2 years        
Vested and outstanding awards   2,166   2,166  
Shares          
Issued (in shares) 1,183,400     1,183,000  
Vested (in shares)       2,000  
Forfeited and canceled (in shares)       (26,000)  
Unvested at end of period (in shares)   1,154,934   1,154,934  
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   $ 3.63  
Non-cash restricted stock unit award expense recognized   $ 600   $ 800  
Unrecognized compensation related to unvested restricted stock units   $ 3,400   $ 3,400  
Expenses recognized over a weighted-average period (in years)       1 year 7 months 17 days