Stockholder's equity (deficit) - Restricted stock activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of Shares
Beginning balance of unvested restricted common stock	80,190	226,400
Early exercise of unvested stock options	491,207
Vested	(416,248)	(146,210)
Ending balance of unvested restricted common stock	155,149	80,190
Grant-Date Fair Value
Early exercise of unvested stock options	$ 650.0000
Vested	650.0000
Ending balance of unvested restricted common stock	$ 650.0000