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Income Taxes - Components of the Deferred Tax Assets (Details) - USD ($)
$ in Thousands
Dec. 31, 2021
Dec. 31, 2020
Income Taxes    
Operating loss carryforwards $ 90,550 $ 90,895
Tax credit carryforwards 28,226 26,550
Stock-based compensation 6,902 6,820
Capitalized research and development 7,818  
Lease liabilities 220 276
Other 70 162
Total deferred tax assets 133,786 124,703
Right-of-use asset (213) (270)
Valuation allowance $ (133,573) $ (124,433)