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Stock based awards - Stock-based compensation (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 6 Months Ended
Jun. 30, 2019
Apr. 30, 2019
Jun. 30, 2019
Jun. 30, 2019
Stock-based compensation        
Total stock based compensation for options granted     $ 402 $ 617
Total unrecognized compensation cost related to unvested stock-based awards $ 6,702   6,702 $ 6,702
Weighted average period of recognition for unrecognized compensation cost       3 years 4 months 6 days
Proceeds from the early exercise of stock options in exchange for restricted common stock       $ 300
Common Stock        
Stock-based compensation        
Issuance of restricted common stock upon early exercise of stock options (in shares) 487,933 487,933   487,933
Exercise price of common stock purchase $ 0.61 $ 0.61    
Proceeds from the early exercise of stock options in exchange for restricted common stock $ 300 $ 300    
Research and Development Expense        
Stock-based compensation        
Total stock based compensation for options granted     94 $ 176
General and Administrative Expense        
Stock-based compensation        
Total stock based compensation for options granted     $ 308 $ 441