Income taxes - Reconciliation of Expected Income Tax Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Federal tax benefit at statutory rate	$ (13,336)	$ (20,178)	$ (25,772)
State tax benefit, net of federal benefit	(4,201)	(4,325)	(4,422)
Research and development and orphan drug credits	0	(320)	(350)
Uncertain tax positions	(28)	94	(302)
Subpart F Income	3,092	0	0
Permanent adjustments to expenses	142	726	1,779
Stock-based compensation	142	414	901
Return to provision adjustment	(1,080)	(1,103)	(2,450)
Statutory tax rate differential	(330)	1,600	663
Changes in valuation allowance	14,219	21,162	29,642
Other	131	506	311
Total income tax benefit	$ (1,249)	$ (1,424)	$ 0