Derivatives (Gains And Losses On Derivative Instruments) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Unrealized gain/(loss) - regulatory assets and liabilities	$ 162	[1]	$ (61)	[1]	$ 327	[1]	$ 97	[1]
Realized gain/(loss) - cost of electricity	(108)	[2]	(149)	[2]	(383)	[2]	(406)	[2]
Realized gain/(loss) - cost of natural gas	(5)	[2]	(4)	[2]	(32)	[2]	(66)	[2]
Total commodity risk	$ 49		$ (214)		$ (88)		$ (375)

[1] Unrealized gains and losses on commodity risk-related instruments are recorded assets or liabilities, rather than being recorded to the Condensed Consolidated Statements of Income. These amounts exclude the impact of cash collateral postings.

[2] These amounts are fully passed through to customers in rates. Accordingly, net income was not impacted by realized amounts on these instruments.