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Risk Management and Trading Activities - Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Total net derivative contract assets	$ 541	$ 518
Total net derivative contract liabilities	573	516
Discontinued Operations [Member]
Derivative [Line Items]
Total net derivative contract assets	487	483
Total net derivative contract liabilities	$ 628	$ 565

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

+ References+ Details

- Definition

Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

+ References+ Details