Commitments and Contingencies (Environmental Matters) (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jul. 31, 2011
So2 Emissions Allowances Obtained For Arp And Cair Compliance				$ 57,000,000
Accrued capital expenditures	328,000,000	240,000,000	240,000,000
Percent further emission reduction goals in participating state power plant CO2 emissions	10.00%
Daily Civil Penalties For Non Compliance	37,500
Estimated Costs To Install Cooling Towers	570,000,000
Amount of Expenditures For Cooling Towers Included In Planned Capital Expenditures	65,000,000
Acquisition Cost Of Rggi Allowances	38,000,000
Minimum [Member]
Daily Civil Penalties For Non Compliance	25,000
Maximum [Member]
Daily Civil Penalties For Non Compliance	$ 37,500