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	Mar. 31, 2013	Dec. 31, 2012
Assets:
Fair value asset derivatives	$ 563	$ 596
Liabilities:
Fair value liability derivatives	594	679
Level 1 [Member]
Assets:
Fair value asset derivatives	5	27
Liabilities:
Fair value liability derivatives	68	15
Level 2 [Member]
Assets:
Fair value asset derivatives	42	45
Liabilities:
Fair value liability derivatives	16	26
Level 3 [Member]
Assets:
Fair value asset derivatives	530	550
Liabilities:
Fair value liability derivatives	510	638
Margin held [Member]
Assets:
Margin held	$ (14)	$ (26)