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Income Taxes - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2007
Income Tax [Line Items]          
Gross federal R&D tax credit-related deferred tax assets derecognized   $ 6,100,000      
Unrecognized tax benefit   1,475,000 $ 1,539,000 $ 1,314,000  
Tax-related penalties or interest recognized   $ 0 $ 0    
Corporate tax rate   34.00% 34.00%    
Decrease in deferred income tax assets   $ 7,300,000      
Decrease in valuation allowance   7,300,000      
Net effect of tax reform enactment   0      
Scenario Forecast          
Income Tax [Line Items]          
Corporate tax rate 21.00%        
Federal Income Tax          
Income Tax [Line Items]          
Net operating loss carryforwards   54,800,000 $ 68,300,000   $ 108,900,000
Tax credit carryforwards available to offset future federal tax liabilities   $ 100,000      
Credit carryforward, expire period   Dec. 31, 2037      
Net operating loss carryforward,derecognized         $ 22,900,000
Federal Income Tax | Beginning Year          
Income Tax [Line Items]          
Net operating loss carryforwards expiration year   2025      
Federal Income Tax | Ending Year          
Income Tax [Line Items]          
Net operating loss carryforwards expiration year   2038      
State Income Tax          
Income Tax [Line Items]          
Net operating loss carryforwards   $ 41,100,000 $ 46,600,000    
Tax credit carryforwards available to offset future federal tax liabilities   $ 4,400,000      
State Income Tax | Beginning Year          
Income Tax [Line Items]          
Net operating loss carryforwards expiration year   2028      
State Income Tax | Ending Year          
Income Tax [Line Items]          
Net operating loss carryforwards expiration year   2038      
CALIFORNIA          
Income Tax [Line Items]          
Net operating loss carryforwards   $ 37,700,000      
Net operating loss carryforward,derecognized   $ 0