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Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($)
 9 Months Ended 167 Months Ended
Jul. 02, 2018
Sep. 30, 2019
Sep. 30, 2019
Dec. 31, 2018
Dec. 29, 2017
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]          
Entity incorporation date   Nov. 22, 2005      
Restructuring and other attrition, percentage of workforce reduction   70.00%      
Accumulated deficit   $ 389,100,000 $ 389,100,000 $ 350,945,000  
Net proceeds from sale of securities     397,900,000    
Credit facility default amount   300,000 300,000    
Total cash, cash equivalents and marketable securities   82,000,000 82,000,000    
Term Loan [Member]          
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]          
Aggregate principal balance   16,400,000 16,400,000   $ 20,000,000
Loan agreement minimum liquidity required   $ 17,200,000 $ 17,200,000 $ 21,000,000  
Public Offering [Member]          
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]          
Issuance of common stock 9,200,000        
Share price per share $ 7.50        
Net proceeds from public offering, after underwriting discount, commission and offering expenses $ 64,600,000