Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Net operating loss carryforwards	$ 32,281	$ 27,441
Federal research and development tax credits	37,953	34,238
Share-based compensation	627	624
Capital loss carryforwards	41,144	41,399
Deferred rent	2	109
Accrued compensation	192	361
Charitable contributions	24	25
Leasehold improvements and equipment	1,292	1,648
Gross deferred tax assets	113,515	105,845
Less: valuation allowance	(113,515)	(105,272)
Total deferred tax assets	0	573
Deferred tax liability:
Capital gain on PRV	0	(573)
Net deferred tax assets	$ 0	$ 0