Derivative instruments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Derivative instruments [Abstract]
Forward notional volumes of natural gas swap agreements for derivative instruments not designated as hedging instruments in MMBtu's	676,000		676,000
Change in fair value of derivative instruments not designated as hedging instruments	$ 1,900,000	$ 3,900,000	$ 8,500,000	$ 9,000,000
Derivative instruments with credit-risk-related contingent features, net liability position, aggregate fair value	900,000		900,000
Aggregate Fair Value for Cascade and/or Intermountain of Assets Needed for Immediate Settlement if credit-risk-related contingent features were triggered	900,000		900,000
Nonmonetary Notional Forward Amount of natural gas swap and collar agreements in MMBtu, Derivative Instruments Designated as cash flow hedging Instruments	23,300,000		23,300,000
Nonmonetary Notional forward Amount of natural gas basis swap agreements in MMBtu, Derivative Instruments Designated as cash flow hedging Instruments	12,900,000		12,900,000
Nonmonetary Notional forward Amount of oil swap, collar and/or put agreements in Bbl, Derivative Instruments Designated as cash flow hedging Instruments	3,700,000		3,700,000
Oil call option agreement with total forward notional volumes (in Bbl)	184,000		184,000
Gain on the derivative instrument that did not qualify for hedge accounting reported in operating revenues	1,900,000		179,000
Maximum Length of Time Hedged in swap, collar and/or put cash flow hedge agreements in months			30
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months from AOCI into earnings			(2,300,000)
Derivative instruments with credit-risk-related contingent features, Net Liability Position, Aggregate Fair Value for Fidelity	24,500,000		24,500,000
Assets Needed for Immediate Settlement if credit-risk-related contingent feature triggered, Aggregate Fair Value for Fidelity	24,500,000		24,500,000
Derivatives, Fair Value [Line Items]
Asset Derivatives Designated as Hedges	20,305,000	33,456,000	20,305,000	33,456,000	19,227,000
Asset Derivatives Not Designated as Hedges	194,000	0	194,000	0	0
Total Asset Derivatives	20,499,000	33,456,000	20,499,000	33,456,000	19,227,000
Liability Derivatives Designated as Hedges	24,515,000	1,961,000	24,515,000	1,961,000	21,552,000
Liability Derivatives Not Designated as Hedges	906,000	18,897,000	906,000	18,897,000	9,359,000
Total Liability Derivatives	25,421,000	20,858,000	25,421,000	20,858,000	30,911,000
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives Designated as Hedges	6,265,000	8,524,000	6,265,000	8,524,000	4,104,000
Asset Derivatives Not Designated as Hedges	0	0	0	0	0
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives Designated as Hedges	6,735,000	0	6,735,000	0	6,483,000
Liability Derivatives Not Designated as Hedges	0	698,000	0	698,000	0
Commodity derivative instruments - current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives Designated as Hedges	14,040,000	24,932,000	14,040,000	24,932,000	15,123,000
Asset Derivatives Not Designated as Hedges	194,000	0	194,000	0	0
Commodity derivative instruments - current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives Designated as Hedges	17,780,000	1,961,000	17,780,000	1,961,000	15,069,000
Liability Derivatives Not Designated as Hedges	$ 906,000	$ 18,199,000	$ 906,000	$ 18,199,000	$ 9,359,000