Derivative Instruments (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Gains (losses)	$ (2.9)	$ (5.0)	$ (12.3)	$ (17.7)
Long-term portion of derivative assets	0		0
Long-term portion of derivative liabilities	0.5		0.5
Derivative Instruments (Textuals)
Regulatory assets related to derivatives	20.7		20.7		22.0
Regulatory liabilities related to derivatives	16.3		16.3		15.3
Collateral in margin accounts	12.8		12.8		11.7
Natural Gas Reserves [Member]
Derivatives, Fair Value [Line Items]
Volume Of Derivative Instruments	15.7 million Dth	17.4 million Dth	54.0 million Dth	65.0 million Dth
Gains (losses)	(10.5)	(8.8)	(27.7)	(33.3)
Natural Gas Reserves [Member] | Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	3.3		3.3		2.5
Natural Gas Reserves [Member] | Other Deferred Credits and Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability	10.2		10.2		11.6
Power [Member]
Derivatives, Fair Value [Line Items]
Volume Of Derivative Instruments	zero MWh	65,040 MWh	zero MWh	224,640 MWh
Gains (losses)	0	(0.5)	0	(0.5)
Oil Reserves [Member]
Derivatives, Fair Value [Line Items]
Volume Of Derivative Instruments	2.2 million gallons	2.3 million gallons	8.8 million gallons	6.0 million gallons
Gains (losses)	2.4	(0.1)	4.9	(0.1)
Oil Reserves [Member] | Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	0.5		0.5		4.4
Oil Reserves [Member] | Other Deferred Credits and Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability	0		0		0
Ftrs [Member]
Derivatives, Fair Value [Line Items]
Volume Of Derivative Instruments	5,896 MW	6,584 MW	18,439 MW	18,673 MW
Gains (losses)	5.2	4.4	10.5	16.2
Ftrs [Member] | Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	10.1		10.1		5.9
Ftrs [Member] | Other Deferred Credits and Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability	0		0		0
Coal [Member] | Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	0.6		0.6		2.9
Coal [Member] | Other Deferred Credits and Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability	0		0		0
Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	14.5		14.5		15.7
Other Deferred Credits and Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liability	$ 10.2		$ 10.2		$ 11.6