Risk Management and Fair Values (Details 1) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
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)	$ 168	$ 220	$ 322
Electricity forwards, swaps and options [Member] | Not Designated as Hedging Instrument [Member]
Effect of Derivative instruments not designated as hedging instruments on the consolidated statements of income
Amount of gain (loss) recognized in OCI	1	15
Electricity forwards, 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 OCI (effective portion)	296	206	315
Competitive businesses operating revenues [Member] | Cash Flow Hedging [Member]
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)	168	220	322
Competitive businesses operating revenues [Member] | Electricity forwards, swaps and options [Member] | Not Designated as Hedging Instrument [Member]
Effect of Derivative instruments not designated as hedging instruments on the consolidated statements of income
Amount of gain (loss) recorded in income	11
Fuel, fuel related expenses and gas purchased for resale [Member] | Natural gas swaps [Member] | Not Designated as Hedging Instrument [Member]
Effect of Derivative instruments not designated as hedging instruments on the consolidated statements of income
Amount of gain (loss) recorded in income	$ (62)	$ (95)	$ (160)