Derivatives and Other Financial Instruments - Schedule of Derivative Contract Assets and Liabilities that are Measured and Recognized at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Assets:
Fair value asset derivatives	$ 422	$ 596	$ 596
Liabilities:
Fair value liability derivatives	516	679
Level 1 [Member]
Assets:
Fair value asset derivatives	8	27
Liabilities:
Fair value liability derivatives	55	15
Level 2 [Member]
Assets:
Fair value asset derivatives	36	45
Liabilities:
Fair value liability derivatives		26
Level 3 [Member]
Assets:
Fair value asset derivatives	385	550
Liabilities:
Fair value liability derivatives	479	638
Margin Held [Member]
Assets:
Margin held	(7)	(26)
Margin Posted [Member]
Liabilities:
Margin posted	$ (18)