Income Taxes - Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
NOLs	$ 25,949	$ 22,033
Research and development credits	14,920	14,138
Research and development expenses capitalized for tax purposes	93,812	92,350
Equity-based compensation	4,130	4,586
Deferred rent	772	847
Depreciation	246	162
Other temporary differences	730	538
Total deferred tax assets	140,559	134,654
Deferred tax liabilities:
Prepaid expenses	(119)	(86)
Total deferred tax liabilities	(119)	(86)
Net deferred tax assets	140,440	134,568
Less valuation allowance	(140,440)	(134,568)
Net deferred tax asset	$ 0	$ 0