Risk Management and Fair Values (Derivative Instruments Designated as Cash Flow Hedges On Consolidated Statements Of Income) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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)	$ 268	$ 168	$ 220
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)	111	296	206
Amount of gain reclassified from accumulated OCI into income (effective portion)	$ 268	$ 168	$ 220