Derivative Instruments (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 MMBTU bbl	Dec. 31, 2010
Derivative Instruments [Line Items]
Natural gas swap agreement with total foward notional volumes	305,000
Change In Fair Value Of Derivative Instruments Not Designated As Hedging Instruments Recorded As Increase Decrease To Regulatory Assets	$ 8,900,000	$ 18,500,000
Aggregate fair value of Assets Needed for Immediate Settlement if credit-risk related contingent features were triggered	437,000
Derivative instruments with credit-risk-related contingent features, net liability position, aggregate fair value	437,000
Nonmonetary Notional Forward Amount of natural gas swap, collar and/or put agreements in MMBtu, Derivative Instruments Designated as cash flow hedging Instruments	10,800,000
Notional Forward Amount Of Cash Flow Hedge Instruments Natural Gas Basis Swaps	3,500,000
Notional Forward Amount Of Cash Flow Hedge Instruments Oil Swaps And Collars	4,000,000
Notional Amount of Derivatives	60,000,000
Maximum length of time hedged in swap, collar and/or put cash flow hedge agreements (in months)	24
Amount of Hedge Ineffectiveness	1,800,000
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings	8,700,000
Derivative Net Liability Position Aggregate Fair Value For Cash Flow Hedging Instruments	18,400,000
Assets Needed For Immediate Settlement Aggregate Fair Value For Cash Flow Hedging Instruments	$ 18,400,000