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Income Taxes (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Reconciliation of federal statutory income tax rate to effective income tax rate      
Federal statutory income tax rate $ (6,157) $ (5,583) $ (20,488)
Loss on settlement of notes     18,884
Non-deductible research and development credit 708 435  
Stock based compensation 651 478  
Other 14 2 4
Net change in valuation allowance 4,784 4,668 $ 1,600
Deferred income tax assets      
Net operating loss carryforwards 69,348 47,401  
Research credits 8,096 12,789  
Depreciation 735 790  
Stock-based compensation 206    
Accrued compensation 511 373  
Deferred revenue 1,032 1,836  
Gross deferred income tax assets 79,928 63,189  
Valuation allowance (76,036) (59,602)  
Net deferred income tax assets 3,892 3,587  
Deferred income tax liabilities      
Patent costs (3,489) (3,199)  
Licensing costs (339) (317)  
Capitalized legal costs (64) (71)  
Gross deferred income tax liabilities $ (3,892) $ (3,587)  
Net deferred income tax asset/(liability)  
Increase (decrease) in deferred tax asset valuation allowance $ (21,400)