Quarterly report pursuant to Section 13 or 15(d)

Licenses Acquired - license as recorded in the Condensed Consolidated Statements of Operations (Detail)

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Licenses Acquired - license as recorded in the Condensed Consolidated Statements of Operations (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Research and Development Arrangement, Contract to Perform for Others [Line Items]    
Research and Development in Process $ 450 $ 97
Mustang Therapeutics, Inc [Member]    
Research and Development Arrangement, Contract to Perform for Others [Line Items]    
Research and Development in Process 450 75
Mustang Therapeutics, Inc [Member] | City of Hope COH CD123 [Member]    
Research and Development Arrangement, Contract to Perform for Others [Line Items]    
Research and Development in Process 250 0
Mustang Therapeutics, Inc [Member] | City Of Hope Manufacturing [Member]    
Research and Development Arrangement, Contract to Perform for Others [Line Items]    
Research and Development in Process 0 75
Mustang Therapeutics, Inc [Member] | Nationwide Childrens Hospital [Member]    
Research and Development Arrangement, Contract to Perform for Others [Line Items]    
Research and Development in Process $ 200 $ 0