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Significant Accounting Policies - Carrying Amounts and Estimated Fair Values of Debt from Royalty-Bearing Instrument (Detail) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Debt from royalty-bearing instrument, carrying value $ 22,266 $ 49,702
Debt from Royalty-bearing Instrument    
Debt Instrument [Line Items]    
Debt from royalty-bearing instrument, estimated fair value $ 22,500 $ 50,400