Commitments and Contingencies (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Oct. 31, 2011
CAA authorized 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 through 2012	65,000,000
Percent further emmision reduction goals in participating state power plant CO2 emissions	10.00%
Acquisition Cost Of Rggi Allowances	38,000,000
Exposure under guarantees	4,435,000,000
Purchased surety bonds	155,000,000
Authorized issuance of standby letters of credit	36,000,000
Third Party and Equity Method Investee
Issued guarantees	77,000,000
Exposure under guarantees	45,000,000
Minimum [Member]
CAA authorized daily civil penalties for non compliance	25,000
Maximum [Member]
CAA authorized daily civil penalties for non compliance	37,500
MF Global Bankruptcy
Initial margin posted		73,000,000
Margin received	$ 8,000,000