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Income Taxes - Deferred tax assets and liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Deferred income tax assets    
Net operating loss carryforwards $ 49,889 $ 25,565
Research credits 23,151 16,642
Depreciation 207 437
Unrealized loss on securities 269 504
Accrued compensation 1,097 748
Deferred revenue 11,222 16,820
State taxes   (2)
Gross deferred income tax assets 85,835 60,714
Valuation allowance (82,537) (57,663)
Net deferred income tax assets 3,298 3,051
Deferred income tax liabilities    
Patent costs (3,142) (2,873)
Licensing costs (125) (142)
Capitalized legal costs (31) (36)
Gross deferred income tax liabilities $ (3,298) $ (3,051)