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	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2012
Assets:
Fair value asset derivatives	$ 371	$ 390	$ 596
Liabilities:
Fair value liability derivatives	440	479
Level 1 [Member]
Assets:
Fair value asset derivatives	7	6
Liabilities:
Fair value liability derivatives	63	66
Level 2 [Member]
Assets:
Fair value asset derivatives	31	32
Level 3 [Member]
Assets:
Fair value asset derivatives	337	355
Liabilities:
Fair value liability derivatives	404	431
Margin Held [Member]
Assets:
Margin held	(4)	(3)
Margin Posted [Member]
Liabilities:
Margin posted	$ (27)	$ (18)