Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative asset	$ 31.2		$ 31.2		$ 20.6
Derivative liability	2.1		2.1		9.2
Gains (losses)	(2.3)	(2.9)	(29.1)	(12.3)
Derivative Instruments (Textuals)
Collateral in margin accounts	0.4		0.4		11.9
Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative assets	1.6		1.6
Other Deferred Credits and Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative liabilities	0.2		0.2
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	12.0		12.0		2.1
Derivative liability	0		0		9.1
Volume of derivative instruments	17.2 million Dth	15.7 million Dth	56.0 million Dth	54.0 million Dth
Gains (losses)	(5.3)	(10.5)	(35.7)	(27.7)
Fuel Oil [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	0.9		0.9		0.3
Derivative liability	0		0		0.1
Volume of derivative instruments	1.7 million gallons	2.2 million gallons	5.5 million gallons	8.8 million gallons
Gains (losses)	0.1	2.4	1.5	4.9
Ftrs [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	7.3		7.3		5.7
Derivative liability	0		0		0
Volume of derivative instruments	5,927 MW	5,896 MW	16,581 MW	18,439 MW
Gains (losses)	2.9	5.2	5.1	10.5
Coal [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	11.0		11.0		12.5
Derivative liability	$ 2.1		$ 2.1		$ 0