Annual report pursuant to Section 13 and 15(d)

Income Taxes - Additional Information (Details)

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Income Taxes - Additional Information (Details) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Taxes [Line Items]      
Increase in valuation allowance $ 900,000    
Unrecognized tax benefits 734,000 $ 620,000 $ 574,000
Interest or penalties accrued $ 0    
Corporate income tax rate 21.00%    
Provisional income tax expense (benefit)   $ 9,800,000  
Estimated re-measurement of deferred tax asset $ 9,800,000    
California      
Income Taxes [Line Items]      
Research and experimental credits carryforwards $ 1,500,000    
Research and experimental credits carryforwards, expiration The California research and experimental tax credits carry forward indefinitely until utilized.    
Federal      
Income Taxes [Line Items]      
Net operating loss carryforwards $ 85,400,000    
Net operating loss carryforward subject to expiration 80,600,000    
Net operating loss carryforward not subject to expiration $ 4,800,000    
Net operating loss carryforwards, earliest expiration year 2026    
Net operating loss carryforwards latest expiration year 2037    
Research and experimental credits carryforwards $ 1,500,000    
Research and experimental credits carryforwards, expiration The federal research and experimental tax credits will begin to expire, if not utilized, in 2026.    
State      
Income Taxes [Line Items]      
Net operating loss carryforwards $ 54,300,000    
Net operating loss carryforwards, earliest expiration year 2019    
Net operating loss carryforwards latest expiration year 2038