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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]      
Valuation allowance $ 40,290,000 $ 26,515,000 $ 13,060,000
Description of tax benefit likely to be realized upon settlement greater than 50%    
Unrecognized tax benefits $ 0    
Income tax examination description In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. Tax years from 2015 to the present are open to examination under the statute.    
Interest or penalties accrued $ 0 0  
Income tax cuts and jobs act provisional amounts measurement period 1 year    
Adjustment of deferred tax asset and liabilities due to tax cuts and jobs act $ 0    
Net impact of revaluation of deferred tax assets and liabilities 0    
Federal      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards $ 125,500,000 $ 82,000,000  
Net operating loss carryforwards expiration year 2034 2034  
Federal | Research and Development.      
Income Tax Disclosure [Line Items]      
Tax credit carryforwards $ 2,800,000    
Tax credit carryforwards, expiration year 2034    
State      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards $ 125,500,000 $ 82,000,000  
Net operating loss carryforwards expiration year 2034 2034  
State | Research and Development.      
Income Tax Disclosure [Line Items]      
Tax credit carryforwards $ 1,600,000    
Tax credit carryforwards, expiration year 2029