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Derivatives, Hedges, Financial Instruments and Carbon Credits - Assets and Liabilities Measured at Fair Value on Recurring Basis (Parenthetical) (Detail) - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
Recurring [Member] | Commodities Contracts [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Commodities contracts $ 2 $ 195