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	Jun. 30, 2013	Dec. 31, 2012
Assets:
Fair value asset derivatives	$ 470	$ 596
Liabilities:
Fair value liability derivatives	504	679
Level 1 [Member]
Assets:
Fair value asset derivatives	6	27
Liabilities:
Fair value liability derivatives	105	15
Level 2 [Member]
Assets:
Fair value asset derivatives	35	45
Liabilities:
Fair value liability derivatives		26
Level 3 [Member]
Assets:
Fair value asset derivatives	432	550
Liabilities:
Fair value liability derivatives	434	638
Margin Held [Member]
Assets:
Margin held	(3)	(26)
Margin Posted [Member]
Liabilities:
Margin posted	$ (35)