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	Mar. 31, 2017	Dec. 31, 2016
Schedule Of Carrying Values And Estimated Fair Values [Line Items]
Carrying Amount	$ 375	$ 375
Estimated Fair Value	$ 366	$ 356