Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative asset	$ 18.5		$ 18.5		$ 19.2
Derivative liability	1.7		1.7		1.9
Gains (losses)	6.7	(11.8)	1.2	(26.8)
Derivative Instruments (Textuals)
Collateral in margin accounts	3.0		3.0		2.9
Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative assets	2.0		2.0
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	2.1		2.1		3.0
Derivative liability	1.5		1.5		1.9
Volume of derivative instruments	11.3 million Dth	18.5 million Dth	30.1 million Dth	38.9 million Dth
Gains (losses)	1.9	(14.2)	(4.6)	(30.4)
Fuel Oil [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	0.1		0.1		0.4
Derivative liability	0.2		0.2		0
Volume of derivative instruments	2.2 million gallons	2.1 million gallons	3.7 million gallons	3.8 million gallons
Gains (losses)	0.1	0.8	0.2	1.4
Ftrs [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	9.2		9.2		4.7
Derivative liability	0		0		0
Volume of derivative instruments	5,744 MW	5,296 MW	11,088 MW	10,654 MW
Gains (losses)	4.7	1.6	5.6	2.2
Coal [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	7.1		7.1		11.1
Derivative liability	$ 0		$ 0		$ 0