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Income Taxes - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Line Items]      
Income tax expense $ 0 $ 0 $ 0
Increase (decrease) in valuation allowance $ 40,400,000 (108,800,000)  
Federal corporate income tax rate 21.00%    
Unrecognized tax benefits $ 1,229,000 $ 1,229,000  
Unrecognized tax benefits would affect the effective tax rate 0    
Interest related to unrecognized tax benefits 0    
Penalties related to unrecognized tax benefits 0    
Federal      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards $ 771,800,000    
Federal | Minimum      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards, expiration year 2019    
Federal | Research And Development Tax Credits      
Income Tax Disclosure [Line Items]      
Tax credit carryforward amount $ 19,900,000    
Federal | Research And Development Tax Credits | Minimum      
Income Tax Disclosure [Line Items]      
Tax credit carryforward amount, expiration year 2019    
Federal | Research And Development Tax Credits | Maximum      
Income Tax Disclosure [Line Items]      
Tax credit carryforward amount, expiration year 2038    
California and Other States      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards $ 229,000,000    
California and Other States | Minimum      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards, expiration year 2019    
California and Other States | Maximum      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards, expiration year 2038    
California State | Research And Development Tax Credits      
Income Tax Disclosure [Line Items]      
Tax credit carryforward amount $ 19,100,000    
Foreign      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards $ 11,900,000