Income Taxes - Deferred tax assets (liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Net operating loss carryforwards	$ 48,535	$ 47,079
Tax credit carryforwards	10,388	9,848
Share-based compensation	7,071	8,354
Property and equipment	0	1
Capitalized research and development	16,440	17,399
Accruals and reserves	2	340
Total deferred tax assets	82,436	83,021
Deferred tax liabilities:
Operating lease - right of use asset	0	(333)
Prepaid sublease termination	0	(658)
Total deferred tax liabilities	0	(991)
Net deferred tax assets	82,436	82,030
Valuation allowance	(82,436)	(82,030)
Net deferred tax assets	$ 0	$ 0