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Significant Accounting Policies - Additional Information (Detail)
12 Months Ended
May 05, 2016
Program
Dec. 31, 2017
USD ($)
Segment
Mar. 31, 2018
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Significant Accounting Policies [Line Items]            
Number of operating segment | Segment   1        
Cash and cash equivalents   $ 142,503,000   $ 150,293,000 $ 161,220,000 $ 1,048,000
Restricted cash   3,610,000   3,601,000    
Impairment charges   $ 0        
Expected dividend rate   0.00%        
Description of Income tax benefit, likelihood of realized upon ultimate settlement   The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement.        
Current portion of deferred revenue   $ 2,705,000   $ 2,705,000    
Accounting Standards Update No. 2016-16 [Member] | Scenario Forecast [Member]            
Significant Accounting Policies [Line Items]            
Cumulative-effect adjustment to retained earnings     $ 400,000      
Minimum [Member]            
Significant Accounting Policies [Line Items]            
Income tax examination, likelihood of settlement, percentage   50.00%        
Pfizer Inc. [Member]            
Significant Accounting Policies [Line Items]            
Number of research programs, counterparty nomination | Program 5          
Revenue recognized   $ 5,200,000        
Deferred revenue   8,300,000        
Current portion of deferred revenue   2,700,000        
Lexington, MA [Member]            
Significant Accounting Policies [Line Items]            
Restricted cash   2,600,000        
Cambridge MA [Member]            
Significant Accounting Policies [Line Items]            
Restricted cash   $ 1,000,000