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Derivatives, Hedges, Financial Instruments and Carbon Credits - Schedule of Carrying Values and Estimated Fair Values of Financial Instruments (Detail) - Senior Secured Notes Due 2019 [Member] - USD ($)
$ in Millions
Sep. 30, 2016
Dec. 31, 2015
Schedule Of Carrying Values And Estimated Fair Values [Line Items]    
Carrying Amount $ 425 $ 425
Estimated Fair Value $ 430 $ 355