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Liability Related to Sale of Future Royalties - Additional Information (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 70 Months Ended
Feb. 24, 2012
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Liability Related to Sale of Future Royalties [Line Items]            
Non-cash royalty revenue related to sale of future royalties       $ 24,337 $ 21,367  
Non-cash interest expense on liability related to sale of future royalties   $ 4,814 $ 4,471 $ 14,808 $ 13,535  
Annual interest rate       17.60%   17.00%
Prospective interest rate       21.00%    
Purchase and Sale Agreement with RPI [Member]            
Liability Related to Sale of Future Royalties [Line Items]            
Proceeds from sale of royalty rights $ 124,000          
Transaction costs related to sale of potential future royalties $ 4,400