Risk Management and Fair Values (Derivative Instruments Designated as Cash Flow Hedges On Consolidated Statements Of Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Effect of Derivative instruments designated as cash flow hedges on consolidated statements of income
Amount of gain reclassified from accumulated OCI into income (effective portion)	$ (2)	$ 71
Competitive Businesses Operating Revenues [Member] | Electricity Swaps And Options [Member] | Cash Flow Hedging [Member]
Effect of Derivative instruments designated as cash flow hedges on consolidated statements of income
Amount of gain (loss) recognized in AOCI (effective portion)	(120)	291
Amount of gain reclassified from accumulated OCI into income (effective portion)	$ (2)	$ 71