Derivative Accounting (Details 3) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Balance Sheet Total	$ 38,897,000	$ 73,788,000
Balance Sheet Total	43,173,000	39,032,000
Balance Sheet Total	(58,684,000)	(58,976,000)
Balance Sheet Total	(57,073,000)	(65,390,000)
Designated accounting hedging relationships | Commodity Contracts
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	15,070,000	15,351,000
Liabilities	(147,946,000)	(181,428,000)
Total derivatives	(132,876,000)	(166,077,000)
Designated accounting hedging relationships | Commodity Contracts | Current Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	329,000	1,234,000
Liabilities	(550,000)	(602,000)
Total derivatives	(221,000)	632,000
Designated accounting hedging relationships | Commodity Contracts | Investments and Other Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets		142,000
Liabilities		(1,933,000)
Total derivatives		(1,791,000)
Designated accounting hedging relationships | Commodity Contracts | Current Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	11,730,000	9,062,000
Liabilities	(90,187,000)	(107,784,000)
Total derivatives	(78,457,000)	(98,722,000)
Designated accounting hedging relationships | Commodity Contracts | Deferred Credits and Other
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	3,011,000	4,913,000
Liabilities	(57,209,000)	(71,109,000)
Total derivatives	(54,198,000)	(66,196,000)
Not designated as accounting hedges | Commodity Contracts
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	110,091,000	124,966,000
Liabilities	(178,066,000)	(198,353,000)
Total derivatives	(67,975,000)	(73,387,000)
Not designated as accounting hedges | Commodity Contracts | Current Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	24,607,000	36,831,000
Liabilities	(869,000)	(312,000)
Total derivatives	23,738,000	36,519,000
Not designated as accounting hedges | Commodity Contracts | Investments and Other Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	43,173,000	40,927,000
Liabilities		(33,000)
Total derivatives	43,173,000	40,894,000
Not designated as accounting hedges | Commodity Contracts | Current Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	31,946,000	27,322,000
Liabilities	(94,294,000)	(112,535,000)
Total derivatives	(62,348,000)	(85,213,000)
Not designated as accounting hedges | Commodity Contracts | Deferred Credits and Other
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	10,365,000	19,886,000
Liabilities	(82,903,000)	(85,473,000)
Total derivatives	(72,538,000)	(65,587,000)
Commodity Contracts
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(200,851,000)	(239,464,000)
Margin account	17,086,000	25,576,000
Collateral provided to counterparties	157,470,000	203,316,000
Collateral provided from counterparties	(12,145,000)	(3,000,000)
Prepaid option premiums and other	4,753,000	2,026,000
Balance Sheet Total	(33,687,000)	(11,546,000)
Credit Risk and Credit-Related Contingent Features
Concentration of credit risk, number of counterparties	2
Concentration of risk with two counterparties, as a percentage of risk management assets (as a percent)	64.00%
Risk management assets	82,000,000
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that were in a liability position	303,000,000
Posted collateral	147,000,000
Additional collateral to counterparties for derivative instruments in a net liability position	106,000,000
Additional collateral to counterparties for energy related non-derivative instrument contracts	194,000,000
Commodity Contracts | Current Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	23,517,000	37,151,000
Margin account	2,137,000	24,579,000
Collateral provided to counterparties	10,000,000	11,556,000
Collateral provided from counterparties		(1,750,000)
Prepaid option premiums and other	3,243,000	2,252,000
Balance Sheet Total	38,897,000	73,788,000
Commodity Contracts | Investments and Other Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	43,173,000	39,103,000
Prepaid option premiums and other		(71,000)
Balance Sheet Total	43,173,000	39,032,000
Commodity Contracts | Current Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(140,805,000)	(183,935,000)
Margin account	11,087,000	997,000
Collateral provided to counterparties	81,669,000	125,367,000
Collateral provided from counterparties	(12,145,000)	(1,250,000)
Prepaid option premiums and other	1,510,000	(155,000)
Balance Sheet Total	(58,684,000)	(58,976,000)
Commodity Contracts | Deferred Credits and Other
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(126,736,000)	(131,783,000)
Margin account	3,862,000
Collateral provided to counterparties	65,801,000	66,393,000
Balance Sheet Total	$ (57,073,000)	$ (65,390,000)