Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Derivatives Fair Value [Line Items]
Derivative, Asset	$ 17.9	$ 20.6
Derivative, Liability	0.5	9.2
Gains (losses)	(28.4)	(14.0)
Other Deferred Charges and Other Assets [Member]
Derivatives Fair Value [Line Items]
Long-term portion of derivative assets	0.6
Other Long-Term Liabilities [Member]
Derivatives Fair Value [Line Items]
Long-term portion of derivative liabilities	0
Natural Gas [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	1.7	2.1
Derivative, Liability	0.5	9.1
Volume	77.2 million Dth	71.8 million Dth
Gains (losses)	(36.3)	(33.4)
Fuel Oil [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	0.4	0.3
Derivative, Liability	0	0.1
Volume	7.0 million gallons	13.0 million gallons
Gains (losses)	1.8	6.9
FTRs [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	4.7	5.7
Derivative, Liability	0	0
Volume	20,616 MW	23,718 MW
Gains (losses)	6.1	12.5
Coal [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	11.1	12.5
Derivative, Liability	$ 0	$ 0