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Derivatives, Hedges, Financial Instruments and Carbon Credits - Schedule of Carrying Values and Estimated Fair Values of Financial Instruments (Parenthetical) (Detail) - $ / shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Derivative Instruments And Hedging Activities Disclosure [Abstract]    
Notes based on quoted price $ 94.88 $ 83.65