Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative asset	$ 11.9		$ 19.2
Derivative liability	0.3		1.9
Gains (losses)	(5.5)	(15.0)
Derivative Instruments (Textuals)
Collateral in margin accounts	0.2		2.9
Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative assets	0.1
Other Deferred Credits and Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative liabilities	0.3
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	6.7		3.0
Derivative liability	0		1.9
Volume of derivative instruments	18.8 million Dth	20.4 million Dth
Gains (losses)	(6.5)	(16.2)
Fuel Oil [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	0.5		0.4
Derivative liability	0		0
Volume of derivative instruments	1.6 million gallons	1.7 million gallons
Gains (losses)	0.1	0.6
Ftrs [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	1.5		4.7
Derivative liability	0		0
Volume of derivative instruments	5,343 MW	5,358 MW
Gains (losses)	0.9	0.6
Coal [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	3.2		11.1
Derivative liability	$ 0.3		$ 0