Risk Management and Derivative Financial Instruments (Details 2) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Electric | Commodity contracts | Derivatives in Cash Flow Hedging Relationships
Gain / (Loss) on derivatives
Amount of Gain / (Loss) Reclassed from OCI into Income (Effective portion)		$ (950,000)		$ (950,000)	$ (4,864,000)	$ (7,853,000)
Amount of Gain / (Loss) Recognized in OCI on Derivative (Effective portion)		676,000		(5,325,000)	1,037,000	(7,434,000)
Electric | Derivatives in Cash Flow Hedging Relationships
Gain / (Loss) on derivatives
Amount of Gain / (Loss) Reclassed from OCI into Income (Effective portion)		(950,000)		(950,000)	(4,864,000)	(7,853,000)
Amount of Gain / (Loss) Recognized in OCI on Derivative (Effective portion)		676,000		(5,325,000)	1,037,000	(7,434,000)
Electric | Commodity contracts
Gain / (Loss) on derivatives
Amount of (Loss) on Derivative recognized as Regulatory (assets)/liabilities	(1,367,000)	(445,000)	(735,000)	(1,737,000)	(2,959,000)	(1,938,000)
Amount of (Loss) Recognized in Fuel and purchased power expense	(552,000)	(135,000)	(392,000)	(397,000)	(1,008,000)	(1,521,000)
Gas | Commodity contracts
Gain / (Loss) on derivatives
Amount of (Loss) on Derivative recognized as Regulatory (assets)/liabilities	(175,000)	(52,000)	(271,000)	(121,000)	(717,000)	(142,000)
Electric
Gain / (Loss) on derivatives
Unrealized losses applicable to financial instruments which will settle in next twelve months	1,200,000		1,200,000		1,200,000
Amount of (Loss) on Derivative recognized as Regulatory (assets)/liabilities	(1,367,000)	(445,000)	(735,000)	(1,737,000)	(2,959,000)	(1,938,000)
Amount of (Loss) Recognized in Fuel and purchased power expense	(552,000)	(135,000)	(392,000)	(397,000)	(1,008,000)	(1,521,000)
Gas
Gain / (Loss) on derivatives
Amount of (Loss) on Derivative recognized as Regulatory (assets)/liabilities	$ (175,000)	$ (52,000)	$ (271,000)	$ (121,000)	$ (717,000)	$ (142,000)