Derivatives (Additional Cash Collateral The Utility Would Be Required To Post If Its Credit Risk-Related Contingency Features Were Triggered) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Derivatives in a liability position with credit risk-related contingencies that are not fully collateralized	$ (133)		$ (266)
Related derivatives in an asset position	29		59
Collateral posting in the normal course of business related to these derivatives	112		103
Net position of derivative contracts/additional collateral posting requirements	$ 8	[1]	$ (104)	[1]

[1] This calculation excludes the impact of closed but unpaid positions, as their settlement is not impacted by any of the Utility's credit risk-related contingencies.