Income taxes - Current income tax provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Current income tax provision:
Income tax benefit related to operating losses and research and development tax credits	$ 0	$ 0
State	24
Total current income tax provision	24
Deferred income tax benefit:
Federal	(9,400)	(4,199)
State	688	(1,239)
Total deferred income tax benefit	(8,712)	(5,438)
Change in deferred tax asset valuation allowance	(8,712)	$ (5,438)
Total provision for income taxes	$ 24