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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, 2017
Dec. 31, 2016
3.75% Convertible Senior Notes [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Convertible Notes, unamortized discount $ 55.4 $ 69.9
12.500% Senior Secured Notes Due 2022 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Unamortized debt premium and consent fees $ 16.3