Stockholder's equity (deficit) - Restricted stock activity (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($) shares
Number of Shares
Beginning balance of unvested restricted common stock	80,190
Early exercise of unvested stock options	491,207
Vested	(150,331)
Ending balance of unvested restricted common stock	421,066
Grant-Date Fair Value
Early exercise of unvested stock options | $	$ 448
Vested | $	96
Ending balance of unvested restricted common stock | $	$ 352
Restricted stock
Number of Shares
Early exercise of unvested stock options	383,320
Ending balance of unvested restricted common stock	421,066