Derivative Instruments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Derivative asset	$ 22,900,000		$ 22,900,000		$ 11,800,000
Derivative liability	200,000		200,000		300,000
Gains (losses)	5,500,000	6,700,000	20,300,000	1,200,000
Derivative Instruments (Textuals)
Collateral in margin accounts	200,000		200,000		0
Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative assets	0		0
Other Deferred Credits and Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative liabilities	0		0
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	7,400,000		7,400,000		5,600,000
Derivative liability	0		0		100,000
Volume of derivative instruments	9.9 million Dth	11.3 million Dth	24.8 million Dth	30.1 million Dth
Gains (losses)	2,500,000	1,900,000	10,100,000	(4,600,000)
Fuel Oil [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	200,000		200,000		600,000
Derivative liability	0		0		0
Volume of derivative instruments	2.4 million gallons	2.2 million gallons	4.4 million gallons	3.7 million gallons
Gains (losses)	400,000	100,000	600,000	200,000
Ftrs [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	14,100,000		14,100,000		3,500,000
Derivative liability	0		0		0
Volume of derivative instruments	7.4 million MWh	6.5 million MWh	13.1 million MWh	12.3 million MWh
Gains (losses)	2,600,000	4,700,000	9,600,000	5,600,000
Coal [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	1,200,000		1,200,000		2,100,000
Derivative liability	$ 200,000		$ 200,000		$ 200,000