Derivative Instruments (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Derivative asset	$ 24,400,000		$ 24,400,000		$ 11,800,000
Derivative liability	1,600,000		1,600,000		300,000
Gains (losses)	1,200,000	4,200,000	21,500,000	5,400,000
Derivative Instruments (Textuals)
Collateral in margin accounts	1,800,000		1,800,000		0
Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative assets	1,700,000		1,700,000
Other Deferred Credits and Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative liabilities	100,000		100,000
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	8,800,000		8,800,000		5,600,000
Derivative liability	1,600,000		1,600,000		100,000
Volume of derivative instruments	6.3 million Dth	6.3 million Dth	31.1 million Dth	36.4 million Dth
Gains (losses)	(800,000)	(1,100,000)	9,300,000	(5,700,000)
Fuel Oil [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	0		0		600,000
Derivative liability	0		0		0
Volume of derivative instruments	2.6 million gallons	2.5 million gallons	7.0 million gallons	6.2 million gallons
Gains (losses)	0	(100,000)	600,000	100,000
Ftrs [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	10,100,000		10,100,000		3,500,000
Derivative liability	0		0		0
Volume of derivative instruments	6.6 million MWh	6.9 million MWh	19.7 million MWh	19.2 million MWh
Gains (losses)	2,000,000	5,400,000	11,600,000	11,000,000
Coal [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	5,500,000		5,500,000		2,100,000
Derivative liability	$ 0		$ 0		$ 200,000