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Income Taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($)
Dec. 31, 2015
Dec. 31, 2014
Deferred tax assets:    
Capitalized research and development costs $ 75,409,000 $ 66,212,000
Net operating losses 59,319,000 57,261,000
Research and development credits 10,581,000 9,798,000
Stock-based compensation 1,925,000 1,124,000
Capital loss carryover 264,000 254,000
Fixed assets (107,000) 393,000
Other 4,707,000 3,930,000
Gross deferred tax assets 152,098,000 138,972,000
Valuation allowance (152,098,000) (138,972,000)
Total deferred tax assets $ 0 $ 0