Stockholder’s equity (deficit) - Common stock (Details) $ / shares in Units, $ in Thousands		3 Months Ended	9 Months Ended
	Jun. 24, 2019 USD ($) $ / shares shares	Jun. 30, 2019 shares	Sep. 30, 2019 Vote $ / shares shares	Jun. 18, 2019 shares	Dec. 31, 2018 $ / shares shares
Class of Stock
Common stock, shares authorized			150,000,000		75,000,000
Common stock, par value (in dollars per share) | $ / shares			$ 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			28,558,653		238,986
Offering costs | $	$ 2,942
Common stock, shares outstanding			28,558,653		238,986
Common stock were reserved for issuance
Options available for future grant			5,147,034		24,115,510
Options
Common stock were reserved for issuance
Options available for future grant			2,455,931		1,219,461
2018 Stock option and grant plan
Common stock were reserved for issuance
Options available for future grant			2,300,708	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
2019 Employee Stock Purchase Plan
Common stock were reserved for issuance
Options available for future grant			273,869
Redeemable Convertible Preferred 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