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Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Deferred tax assets:    
Net operating loss carryforward $ 210,301 $ 172,113
Research and development credits 42,490 35,670
Stock-based compensation 15,628 11,905
Lease liabilities 3,487  
Other 3,818 3,171
Total gross deferred tax assets 275,724 222,859
Deferred tax liabilities:    
Right-of-use lease assets (3,385)  
Total gross deferred tax liabilities (3,385)  
Valuation allowance $ (272,339) $ (222,859)