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Fair Value of Financial Assets and Liabilities - Summary of Fair Value and Carrying Value of Long-Term Debt (Parenthetical) (Detail) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
2024 Senior Secured Notes [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt Instrument, Unamortized Discount $ 3.3 $ 3.9
2022 Convertible Notes [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt Instrument, Unamortized Discount $ 26.1 $ 35.9