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Financial Instruments - Fair Value of Financial Instruments and Other Non-Financial Assets (Footnote) (Detail) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Derivatives, Fair Value [Line Items]    
Accrued interest expense $ 3,000 $ 15,800
BG Joint Venture | Supply Commitment    
Derivatives, Fair Value [Line Items]    
Discount rate 8.00% 8.00%
Level 3 | Fair Value Asset (Liability)    
Derivatives, Fair Value [Line Items]    
Long-term receivable included in other assets $ 5,004 $ 10,944
Level 3 | BG Joint Venture | Supply Commitment | Fair Value Asset (Liability)    
Derivatives, Fair Value [Line Items]    
Long-term receivable included in other assets $ 5,000 $ 10,900