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Income Taxes - Schedule of Significant Components of Company's Deferred Tax Assets (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Deferred tax assets:    
U.S. and state net operating loss carryforwards $ 84,556 $ 91,584
Stock-based compensation 15,748 17,004
Accruals and other temporary differences 2,386 2,491
Research and development credits 29,186 15,898
Capitalized research and development 2,211 3,344
Total deferred tax assets 134,087 130,321
Less valuation allowance (134,087) (130,321)
Net deferred tax assets $ 0 $ 0