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Income Taxes (Reconciliation of Deferred Tax Assets and Liabilities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Deferred tax assets:    
Net operating loss carryforwards $ 226,911 $ 212,424
Research and development credits 45,853 42,635
Accruals and reserves 8,024 4,774
Stock-based compensation 16,219 14,582
Deferred rent   3,315
ASC842 Operating lease liability 10,837  
Total deferred tax assets 307,844 277,730
Less: Valuation allowance (298,658) (275,540)
Total deferred tax assets: 9,186 2,190
Fixed assets (1,425) (2,190)
ASC842 Operating Lease right-of-use assets (7,761)  
Total deferred tax liabilities (9,186) (2,190)
Net deferred tax assets