Redeemable convertible preferred stock - Convertible Preferred Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019
Temporary Equity
Redeemable convertible preferred stock, shares issued	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
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