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Derivatives, Hedges, Financial Instruments and Carbon Credits - Liabilities Measured at Fair Value on Recurring Basis (Parenthetical) (Detail) - USD ($)
Jun. 30, 2017
Dec. 31, 2016
Recurring [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Assets measured at fair value on a recurring basis $ 0 $ 0