Derivative Instruments (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative asset	$ 15,300,000		$ 15,300,000		$ 15,300,000
Derivative liability	38,700,000		38,700,000		12,500,000
Gains (losses)	(5,000,000)	$ 5,500,000	(10,100,000)	$ 20,300,000
Derivative Instruments (Textuals)
Collateral in margin accounts	26,300,000		26,300,000		11,200,000
Other Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative assets	1,000,000		1,000,000
Other Deferred Credits and Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Long-term portion of derivative liabilities	5,200,000		5,200,000
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	6,200,000		6,200,000		5,000,000
Derivative liability	$ 26,900,000		$ 26,900,000		12,300,000
Volume of derivative instruments	10.0 million Dth	9.9 million Dth	23.3 million Dth	24.8 million Dth
Gains (losses)	$ (5,900,000)	$ 2,500,000	$ (13,000,000)	$ 10,100,000
Fuel Oil [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	100,000		100,000		0
Derivative liability	$ 900,000		$ 900,000		0
Volume of derivative instruments	0.8 million gallons	2.4 million gallons	1.7 million gallons	4.4 million gallons
Gains (losses)	$ 100,000	$ 400,000	$ 0	$ 600,000
Ftrs [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	7,700,000		7,700,000		7,000,000
Derivative liability	$ 0		$ 0		0
Volume of derivative instruments	5.9 million MWh	7.4 million MWh	12.1 million MWh	13.1 million MWh
Gains (losses)	$ 800,000	$ 2,600,000	$ 2,900,000	$ 9,600,000
Coal [Member]
Derivatives, Fair Value [Line Items]
Derivative asset	1,300,000		1,300,000		3,300,000
Derivative liability	$ 10,900,000		$ 10,900,000		$ 200,000