Commitments and Contingent Liabilities, Fuel Contracts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Capacity Payments [Member]
Fuel Contracts [Abstract]
Minimum fuel purchase commitment	$ 1,478.9
Purchased Power Agreements [Abstract]
Payments for capacity for purchase power agreements	325.3	426.7	461.3
Coal [Member]
Fuel Contracts [Abstract]
Minimum fuel purchase commitment	3,683.0
Nuclear Fuel [Member]
Fuel Contracts [Abstract]
Minimum fuel purchase commitment	1,546.0
Natural Gas Supply [Member]
Fuel Contracts [Abstract]
Minimum fuel purchase commitment	1,122.0
Natural Gas Storage and Transportation [Member]
Fuel Contracts [Abstract]
Minimum fuel purchase commitment	$ 2,755.0