Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Derivatives Fair Value [Line Items]
Derivative, Asset	$ 20.6	$ 15.7
Derivative, Liability	9.2	11.6
Gains (losses)	(14.0)	(25.6)
Other Deferred Charges and Other Assets [Member]
Derivatives Fair Value [Line Items]
Long-term portion of derivative assets	2.5
Other Long-Term Liabilities [Member]
Derivatives Fair Value [Line Items]
Long-term portion of derivative liabilities	0.7
Power [Member]
Derivatives Fair Value [Line Items]
Volume	zero MWh	234,720 MWh
Gains (losses)	0	(0.5)
FTRs [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	5.7	5.9
Derivative, Liability	0	0
Volume	23,718 MW	25,234 MW
Gains (losses)	12.5	19.2
Coal [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	12.5	2.9
Derivative, Liability	0	0
Fuel Oil [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	0.3	4.4
Derivative, Liability	0.1	0
Volume	13.0 million gallons	8.1 million gallons
Gains (losses)	6.9	(0.5)
Natural Gas [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	2.1	2.5
Derivative, Liability	9.1	11.6
Volume	71.8 million Dth	83.2 million Dth
Gains (losses)	$ (33.4)	$ (43.8)