Derivative Accounting (Details 3) (USD $)	Sep. 30, 2011	Dec. 31, 2010
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	$ 60,029,000	$ 112,820,000
Liabilities	(119,412,000)	(124,366,000)
Balance Sheet Total	27,713,000	73,788,000
Balance Sheet Total	32,316,000	39,032,000
Balance Sheet Total	(60,667,000)	(58,976,000)
Balance Sheet Total	(58,745,000)	(65,390,000)
Balance Sheet Total	59,383,000	(11,546,000)
Designated accounting hedging relationships | Commodity Contracts
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(97,219,000)	(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	6,608,000	10,295,000
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	2,700,000	5,056,000
Designated accounting hedging relationships | Commodity Contracts | Total Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	9,308,000	15,351,000
Designated accounting hedging relationships | Commodity Contracts | Current Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Liabilities	(61,134,000)	(108,387,000)
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
Liabilities	(45,393,000)	(73,041,000)
Designated accounting hedging relationships | Commodity Contracts | Total Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Liabilities	(106,527,000)	(181,428,000)
Not designated as accounting hedges | Commodity Contracts
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(70,707,000)	(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	64,177,000	64,153,000
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	40,918,000	60,813,000
Not designated as accounting hedges | Commodity Contracts | Total Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	105,095,000	124,966,000
Not designated as accounting hedges | Commodity Contracts | Current Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Liabilities	(110,437,000)	(112,847,000)
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
Liabilities	(65,365,000)	(85,506,000)
Not designated as accounting hedges | Commodity Contracts | Total Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Liabilities	(175,802,000)	(198,353,000)
Commodity Contracts
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Margin and Collateral Provided to Counterparties	119,242,000	228,892,000
Collateral provided from counterparties	(12,145,000)	(3,000,000)
Other	1,446,000	2,026,000
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)	76.00%
Risk management assets	60,000,000
Aggregate Fair Value of Derivative Instruments in a Net Liability Position	254,000,000
Cash Collateral Posted	99,000,000
Additional Cash Collateral in the Event Credit-Risk Related Contingent Features were Fully Triggered	136,000,000
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
Margin and Collateral Provided to Counterparties	1,529,000	36,135,000
Collateral provided from counterparties		(1,750,000)
Other	(44,601,000)	(35,045,000)
Commodity Contracts | Investments and Other Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Other	(11,302,000)	(26,837,000)
Commodity Contracts | Total Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Margin and Collateral Provided to Counterparties	1,529,000	36,135,000
Collateral provided from counterparties		(1,750,000)
Other	(55,903,000)	(61,882,000)
Commodity Contracts | Current Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Margin and Collateral Provided to Counterparties	77,002,000	126,364,000
Collateral provided from counterparties	(12,145,000)	(1,250,000)
Other	46,047,000	37,144,000
Commodity Contracts | Deferred Credits and Other
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Margin and Collateral Provided to Counterparties	40,711,000	66,393,000
Other	11,302,000	26,764,000
Commodity Contracts | Total Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Margin and Collateral Provided to Counterparties	117,713,000	192,757,000
Collateral provided from counterparties	(12,145,000)	(1,250,000)
Other	$ 57,349,000	$ 63,908,000