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Income Taxes - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Income Tax Disclosure [Line Items]      
Income tax expense $ 0 $ 0 $ 0
Increase (decrease) in valuation allowance 19,000,000.0 22,300,000  
Unrecognized tax benefits 10,565,000 $ 10,322,000  
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 $ 955,000,000.0    
Federal | Minimum      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards, expiration year 2021    
Federal | Research And Development Tax Credits      
Income Tax Disclosure [Line Items]      
Tax credit carryforward amount $ 22,500,000    
Federal | Research And Development Tax Credits | Minimum      
Income Tax Disclosure [Line Items]      
Tax credit carryforward amount, expiration year 2021    
Federal | Research And Development Tax Credits | Maximum      
Income Tax Disclosure [Line Items]      
Tax credit carryforward amount, expiration year 2040    
California and Other States      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards $ 373,200,000    
California and Other States | Minimum      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards, expiration year 2021    
California and Other States | Maximum      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards, expiration year 2040    
California State | Research And Development Tax Credits      
Income Tax Disclosure [Line Items]      
Tax credit carryforward amount $ 19,800,000    
Foreign      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards $ 6,700,000