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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 [Member] - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Carrying Values And Estimated Fair Values [Line Items]    
Carrying Amount $ 375 $ 375
Estimated Fair Value $ 372 $ 356