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Significant Accounting Policies - Carrying Amounts and Estimated Fair Values of Debt Instruments (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Current portion of long-term debt from royalty-bearing instrument, net of accrued interest, carrying value $ 50,672 $ 33,602
Long-term debt from royalty-bearing instrument, carrying value 8,878 46,108
Total long-term debt from royalty-bearing instrument, carrying value 59,550 79,710
Long-term debt from royalty-bearing instrument    
Debt Instrument [Line Items]    
Total long-term debt from royalty-bearing instrument, estimated fair value $ 60,000 $ 78,600