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Income Taxes - Components of the Deferred Tax Assets (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Income Taxes    
Operating loss carryforwards $ 90,895 $ 81,403
Tax credit carryforwards 26,550 23,072
Stock-based compensation 6,820 7,818
Lease liabilities 276 308
Other 162 176
Total deferred tax assets 124,703 112,777
Right-of-use asset (270) (306)
Valuation allowance $ (124,433) $ (112,471)