Derivative Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Derivative asset	$ 9,200,000		$ 11,800,000
Derivative liability	100,000		300,000
Gains (losses)	14,800,000	(5,500,000)
Derivative Instruments (Textuals)
Collateral in margin accounts	0		0
Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative assets	0
Other Deferred Credits and Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative liabilities	0
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	6,100,000		5,600,000
Derivative liability	0		100,000
Volume of derivative instruments	14.9 million Dth	18.8 million Dth
Gains (losses)	7,600,000	(6,500,000)
Fuel Oil [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	100,000		600,000
Derivative liability	0		0
Volume of derivative instruments	2.0 million gallons	1.6 million gallons
Gains (losses)	200,000	100,000
Ftrs [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	1,700,000		3,500,000
Derivative liability	0		0
Volume of derivative instruments	5.7 million MWh	5.8 million MWh
Gains (losses)	7,000,000	900,000
Coal [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	1,300,000		2,100,000
Derivative liability	$ 100,000		$ 200,000