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Stock-based compensation (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended 84 Months Ended
Oct. 31, 2015
Jan. 31, 2014
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares available for future issuance (in shares)     12,778,375 11,990,940   12,778,375
Stock options granted (in shares)     3,681,075      
Stock-based compensation.     $ 9,344 $ 11,324 $ 14,934  
2014 Stock Option and Grant Plan            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of shares authorized for stock-based compensation plan (in shares)   823,529        
2015 Employee Stock Purchase Plan            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares available for future issuance (in shares)     1,884,309     1,884,309
Percentage of outstanding common stock           1.00%
Common stock shares authorized for issuance           2,692,838
New stock options issued     150,265 200,006    
Number of shares authorized for stock-based compensation plan (in shares) 262,362          
Period to purchase shares     6 months      
2015 Employee Stock Purchase Plan | Minimum            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Percentage of payroll deduction 1.00%          
2015 Employee Stock Purchase Plan | Maximum            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Percentage of payroll deduction 10.00%          
2015 Stock Option Plan            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares available for future issuance (in shares) 1,311,812   3,536,932 4,374,539   3,536,932
Percentage limit of issued and outstanding shares up to which additional shares may be issued 4.00%          
Percentage of outstanding common stock           4.00%
Common stock shares authorized for issuance           10,771,368
Stock options            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares available for future issuance (in shares)     6,199,571 5,013,193   6,199,571
Stock-based compensation.     $ 5,938 $ 7,438 11,387  
Stock options | Inducement Awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     4 years      
Stock options | Inducement Awards | Vesting period, tranche one            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage     25.00%      
Stock options | Inducement Awards | Vesting period, tranche two            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     36 months      
Vesting percentage     75.00%      
Unvested restricted common stock units            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares available for future issuance (in shares)     1,112,563 806,379   1,112,563
Stock-based compensation.     $ 3,215 $ 3,551 $ 3,110  
Restricted stock granted, original issued price (in dollars per share)     $ 4.29      
Unvested restricted common stock units | Vesting period, tranche one            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     3 years      
Unvested restricted common stock units | Vesting period, tranche two            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     4 years      
Unvested restricted common stock units | Inducement Awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
New stock options issued     163,000 13,000 29,000  
Vesting period     3 years      
Unvested restricted common stock units | Inducement Awards | Vesting period, tranche one            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage     33.00%      
Unvested restricted common stock units | Inducement Awards | Vesting period, tranche two            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage     33.00%      
Performance based milestone awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Remaining number of shares authorized to be repurchased     131,470     131,470
Founders restricted stock            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares available for future issuance (in shares)     45,000 137,255   45,000
Restricted stock granted (in shares)   1,188,233        
Restricted stock granted, original issued price (in dollars per share)   $ 0.0425        
Founders restricted stock | Vesting period, tranche one            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Restricted stock granted (in shares)   835,292        
Founders restricted stock | Minimum | Vesting period, tranche one            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period   1 year        
Founders restricted stock | Maximum | Vesting period, tranche one            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period   4 years        
Non-statutory stock options | Inducement Awards            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
New stock options issued     390,000 76,500 172,500  
Employees and directors stock options | 2014 Stock Option and Grant Plan            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     4 years      
Employees and directors stock options | 2014 Stock Option and Grant Plan | Vesting period, tranche one            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     1 year      
Vesting percentage     25.00%      
Employees and directors stock options | 2014 Stock Option and Grant Plan | Vesting period, tranche two            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     3 years      
Vesting percentage     75.00%      
Employees and directors stock options | 2015 Stock Option Plan            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock options granted (in shares)     3,291,075      
Non-employees stock options | 2014 Stock Option and Grant Plan | Minimum            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     1 year      
Non-employees stock options | 2014 Stock Option and Grant Plan | Maximum            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     4 years      
Share-based Payment Arrangement, Employee | Stock options            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock-based compensation.     $ 5,800 $ 7,300 $ 11,200  
Share-based Payment Arrangement, Employee | Unvested restricted common stock units            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock-based compensation.     2,900 3,300 2,800  
Research and development            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock-based compensation.     2,946 4,133 6,357  
General and administrative            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock-based compensation.     $ 6,398 $ 7,191 $ 8,577