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Income Taxes - Deferred tax assets and liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Deferred income tax assets    
Net operating loss carryforwards $ 25,564 $ 54,860
Research credits 16,642 11,561
Depreciation 437 668
Unrealized loss on securities 504 573
Accrued compensation 748 719
Deferred revenue 26,442 5,236
State taxes   97
State taxes (2)  
Gross deferred income tax assets 70,335 73,714
Valuation allowance (67,284) (69,670)
Net deferred income tax assets 3,051 4,044
Deferred income tax liabilities    
Patent costs (2,873) (3,725)
Licensing costs (142) (261)
Capitalized legal costs (36) (58)
Gross deferred income tax liabilities $ (3,051) $ (4,044)