Stockholder's equity (deficit) - Restricted stock activity (Details)	3 Months Ended
	Mar. 31, 2020 USD ($) shares
Number of Shares
Beginning balance of unvested restricted common stock | shares	155,149
Shares vesting | shares	(61,617)
Ending balance of unvested restricted common stock | shares	93,532
Grant-Date Fair Value
Beginning balance of unvested restricted common stock | $	$ 0.52
Shares vesting | $	0.47
Ending balance of unvested restricted common stock | $	$ 0.55