Redeemable convertible preferred stock (Details) - USD ($) $ in Thousands		12 Months Ended
	Jun. 24, 2019	Dec. 31, 2018	Sep. 30, 2019
Temporary Equity
Redeemable convertible preferred stock outstanding		64,730,410	0
Preferred stock authorized		64,730,410	0
Preferred stock issued and outstanding		64,730,410
Carrying value		$ 124,728
Liquidation preference		$ 96,358
Common stock issuable upon conversion		21,056,136
IPO
Temporary Equity
Number of shares converted	21,056,136
Redeemable convertible preferred stock outstanding			0
Series A Preferred Stock
Temporary Equity
Preferred stock authorized		50,858,462
Preferred stock issued and outstanding		50,858,462
Carrying value		$ 79,457
Liquidation preference		$ 50,858
Common stock issuable upon conversion		16,543,739
Series B Preferred Stock
Temporary Equity
Preferred stock authorized		13,871,948
Preferred stock issued and outstanding		13,871,948
Carrying value		$ 45,271
Liquidation preference		$ 45,500
Common stock issuable upon conversion		4,512,397