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License and Asset Purchase Agreements - Additional Information - Biogen Asset Purchase Agreement (Detail) - Biogen MA Inc [Member] - USD ($)
$ in Thousands
3 Months Ended
Jan. 24, 2018
Mar. 31, 2018
Research and Development Arrangement, Contract to Perform for Others [Line Items]    
Revenues   $ 10,000
Asset Purchase Agreement [Member]    
Research and Development Arrangement, Contract to Perform for Others [Line Items]    
Upfront payment received $ 10,000  
Commercial milestone payments receivable 65,000  
Asset Purchase Agreement [Member] | Scenario, Plan [Member] | Maximum [Member] | Sales Milestone [Member]    
Research and Development Arrangement, Contract to Perform for Others [Line Items]    
Milestone payments receivable $ 142,000