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Stockholder's equity (deficit) - Common stock (Details)
$ / shares in Units, $ in Thousands
6 Months Ended
Jun. 30, 2019
$ / shares
shares
Jun. 24, 2019
USD ($)
$ / shares
shares
Jun. 30, 2019
Vote
$ / shares
shares
Jun. 18, 2019
shares
Dec. 31, 2018
$ / shares
shares
Class of Stock          
Common stock, shares authorized 150,000,000   150,000,000   75,000,000
Common stock, par value (in dollars per share) | $ / shares $ 0.0001   $ 0.0001   $ 0.0001
Number of votes per share | Vote     1    
Cash dividend declared | $ / shares 0        
Cash dividend paid | $ / shares $ 0        
Common stock, shares issued 28,555,379   28,555,379   238,986
Offering costs | $   $ 2,942      
Common stock, shares outstanding 28,555,379   28,555,379   238,986
Common stock were reserved for issuance          
Options available for future grant 5,150,308   5,150,308   24,115,510
Options          
Common stock were reserved for issuance          
Options available for future grant 2,455,931   2,455,931   1,219,461
2018 Stock option and grant plan          
Common stock were reserved for issuance          
Options available for future grant 2,303,982   2,303,982 107,635 1,839,913
2019 Stock Option and Incentive plan [Member]          
Common stock were reserved for issuance          
Options available for future grant 116,526   116,526    
2019 Employee Stock Purchase Plan          
Common stock were reserved for issuance          
Options available for future grant 273,869   273,869    
Redeemable convertible preferred stock (as converted to common stock)          
Class of Stock          
Number of shares converted   21,056,136 (64,730,410)    
Common stock were reserved for issuance          
Options available for future grant         21,056,136
IPO          
Class of Stock          
Common stock, shares authorized   150,000,000      
Common stock, par value (in dollars per share) | $ / shares   $ 0.0001      
Common stock, shares issued   6,612,500      
Proceeds received, net of underwriting discounts and commissions | $   $ 98,394      
Number of shares converted   21,056,136      
Over allotment          
Class of Stock          
Public offering price | $ / shares   $ 16.00      
Proceeds received, net of underwriting discounts and commissions | $   $ 98,394      
Over allotment | Maximum          
Class of Stock          
Common stock, shares issued   862,500