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Income Taxes Effect of Each Type of Temporary Difference Comprising Net Differed Tax Asset (Detail) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Deferred tax assets:      
Net operating losses $ 88,996 $ 80,096  
Stock based compensation 17,975 15,600  
Depreciation 74 (90)  
Tax credits 3,076 2,141  
Other reserves and accrued liabilities 2,796 1,708  
Gross deferred tax assets 112,917 99,455  
Less: valuation allowance (94,684) (85,965) $ (88,999)
Total deferred tax assets $ 18,233 $ 13,490