Derivatives and Other Financial Instruments - Schedule of Derivative Contract Assets and Liabilities Measured and Recognized at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Sep. 30, 2012
Assets:
Fair value asset derivatives	$ 346	$ 390	$ 596
Liabilities:
Margin posted	(9)	(18)
Fair value liability derivatives	457	479
Level 1 [Member]
Assets:
Fair value asset derivatives	11	6
Liabilities:
Fair value liability derivatives	30	66
Level 2 [Member]
Assets:
Fair value asset derivatives	30	32
Liabilities:
Fair value liability derivatives
Level 3 [Member]
Assets:
Fair value asset derivatives	305	355
Liabilities:
Fair value liability derivatives	436	431
Margin Held [Member]
Assets:
Margin held		(3)
Margin Posted [Member]
Liabilities:
Margin posted	$ (9)	$ (18)