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Commitments and Contingencies - Additional Information (Detail) - USD ($)
12 Months Ended 36 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Sep. 26, 2016
Apr. 30, 2015
Commitments And Contingencies [Line Items]            
Restricted cash $ 3,625,000 $ 3,610,000   $ 3,625,000    
Lease rent expense $ 4,900,000 5,600,000 $ 1,500,000      
2016 Lexington, MA Lease [Member]            
Commitments And Contingencies [Line Items]            
Lease agreement term         10 years 9 months  
Lease renewal term description 10 year and 9 month lease, which includes two successive five year renewal options          
Restricted cash $ 2,600,000 2,600,000   2,600,000    
Lease extension period         5 years  
Tenant improvement allowances received 8,500,000 2,800,000 $ 100,000 11,400,000    
2015 Cambridge, MA Lease [Member]            
Commitments And Contingencies [Line Items]            
Lease agreement term           7 years 6 months
Restricted cash $ 1,000,000 $ 1,000,000   $ 1,000,000    
Lease extension period           5 years