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Income Taxes - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating Loss Carryforwards [Line Items]      
Valuation allowances recognized in the current year $ 16,400,000 $ 19,300,000  
Effect of tax act on deferred tax assets (38,800,000)    
Effect of Tax Act $ (971,000) $ 0 $ 0
Statutory federal income tax rate 34.00% 34.00% 34.00%
Internal Revenue Code related to annual use of operating loss carry forwards annual use of the Company’s net operating loss and research and development credit carryforwards may be limited in the event a cumulative change in ownership of more than 50% occurs within a rolling three-year period    
Internal Revenue Code ownership percentage change 50.00%    
Income tax benefit recognized related to uncertain tax positions $ 0 $ 0  
Interest expense related to uncertain tax positions 0 0  
Liability related to unrecognized tax benefits 0 0  
Domestic Tax Authority      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforwards $ 351,400,000    
Net operating loss expiration beginning date 2021    
Research and development tax credit carryforwards $ 8,800,000    
Tax credits expiration beginning date 2021    
California Franchise Tax Board      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforwards $ 38,100,000    
Net operating loss expiration beginning date 2027    
Research and development tax credit carryforwards $ 10,400,000    
Foreign Tax Authority      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforwards 38,600,000    
Decrease in valuation allowance of deferred tax assets      
Operating Loss Carryforwards [Line Items]      
Effect of tax act on valuation allowance $ 38,772,000 $ 0 $ 0