Income taxes - Income tax rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income taxes
Federal statutory income tax rate	21.00%	21.00%
State income taxes, net of federal benefit	(1.60%)	1.50%
Research and development tax credits	1.90%	0.10%
Change in preferred stock tranche obligation		(16.00%)
Change in deferred tax asset valuation allowance	(19.90%)	(6.60%)
Other	(1.50%)
Effective income tax rate	(0.10%)	0.00%