Risk Management and Derivative Financial Instruments (Details 2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Electric
Gain / (Loss) on derivatives
Unrealized net losses applicable to financial instruments which will settle in next twelve months	$ 4,200		$ 4,200		$ 4,200
Period within which financial instruments will settle			12 months
Amount of Gain/(Loss) Recognized on Balance Sheet	(1,565)	$ (2,962)	$ 841	$ 10,848	(3,830)	$ 14,255
Amount of Gain / (Loss) Recognized in Income on Derivative	(2,971)	1,828	65	7,965	1,548	7,793
Gas
Gain / (Loss) on derivatives
Amount of Gain/(Loss) Recognized on Balance Sheet	(236)	(23)	(50)	86	(647)	141
Commodity contracts | Electric
Gain / (Loss) on derivatives
Amount of Gain/(Loss) Recognized on Balance Sheet	(1,690)	(2,695)	(3,909)	(537)	(10,152)	903
Amount of Gain / (Loss) Recognized in Income on Derivative	(4,104)	(1,849)	(6,381)	(934)	(7,105)	(1,187)
Commodity contracts | Gas
Gain / (Loss) on derivatives
Amount of Gain/(Loss) Recognized on Balance Sheet	(236)	(23)	(50)	86	(647)	141
Transmission congestion rights (TCR) | Electric
Gain / (Loss) on derivatives
Amount of Gain/(Loss) Recognized on Balance Sheet	125	(267)	4,750	11,385	6,322	13,352
Amount of Gain / (Loss) Recognized in Income on Derivative	$ 1,133	$ 3,677	$ 6,446	$ 8,899	$ 8,653	$ 8,980