Derivative Accounting (Details 3) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	$ 79,586,000	$ 112,820,000
Liabilities	(136,463,000)	(124,366,000)
Current assets	30,264,000	73,788,000
Investments and Other Assets	49,322,000	39,032,000
Current Liabilities	(53,968,000)	(58,976,000)
Deferred Credits and Other	(82,495,000)	(65,390,000)
Derivative Instruments	(56,877,000)	(11,546,000)
Credit Risk and Credit-Related Contingent Features
Risk management assets	79,586,000	112,820,000
Commodity Contracts
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	80,000,000
Margin and Collateral Provided to Counterparties	174,626,000	228,892,000
Collateral provided from counterparties	(11,145,000)	(3,000,000)
Other	1,356,000	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	80.00%
Risk management assets	80,000,000
Aggregate Fair Value of Derivative Instruments in a Liability Position	330,000,000
Cash Collateral Posted	147,000,000
Additional Cash Collateral in the Event Credit-Risk Related Contingent Features were Fully Triggered	151,000,000
Additional collateral to counterparties for energy related non-derivative instrument contracts	194,000,000
Collateral Provided to Counterparties, relating to non-derivative instruments or derivative instruments that qualify for a scope exception		11,000,000
Collateral Provided from Counterparties, relating to non-derivative instruments or derivative instruments that qualify for a scope exception	11,000,000	1,000,000
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	1,630,000	36,135,000
Collateral provided from counterparties		(1,750,000)
Other	(54,815,000)	(35,045,000)
Current assets	30,264,000	73,788,000
Commodity Contracts | Investments and Other Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Other	(12,755,000)	(26,837,000)
Investments and Other Assets	49,322,000	39,032,000
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	1,630,000	36,135,000
Collateral provided from counterparties		(1,750,000)
Other	(67,570,000)	(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	107,228,000	126,364,000
Collateral provided from counterparties	(11,145,000)	(1,250,000)
Other	56,172,000	37,144,000
Current Liabilities	(53,968,000)	(58,976,000)
Commodity Contracts | Deferred Credits and Other
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Margin and Collateral Provided to Counterparties	65,768,000	66,393,000
Other	12,754,000	26,764,000
Deferred Credits and Other	(82,495,000)	(65,390,000)
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	172,996,000	192,757,000
Collateral provided from counterparties	(11,145,000)	(1,250,000)
Other	68,926,000	63,908,000
Commodity Contracts | Designated as Hedging Instruments
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(139,241,000)	(166,077,000)
Commodity Contracts | Designated as Hedging Instruments | Current Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	7,287,000	10,295,000
Commodity Contracts | Designated as Hedging Instruments | Investments and Other Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	3,804,000	5,056,000
Commodity Contracts | Designated as Hedging Instruments | Total Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	11,091,000	15,351,000
Commodity Contracts | Designated as Hedging Instruments | Current Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(82,195,000)	(108,387,000)
Commodity Contracts | Designated as Hedging Instruments | Deferred Credits and Other
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(68,137,000)	(73,041,000)
Commodity Contracts | Designated as Hedging Instruments | Total Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(150,332,000)	(181,428,000)
Commodity Contracts | Not Designated as Hedging Instruments
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(82,473,000)	(73,387,000)
Commodity Contracts | Not Designated as Hedging Instruments | Current Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	76,162,000	64,153,000
Commodity Contracts | Not Designated as Hedging Instruments | Investments and Other Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	58,273,000	60,813,000
Commodity Contracts | Not Designated as Hedging Instruments | Total Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	134,435,000	124,966,000
Commodity Contracts | Not Designated as Hedging Instruments | Current Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(124,028,000)	(112,847,000)
Commodity Contracts | Not Designated as Hedging Instruments | Deferred Credits and Other
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(92,880,000)	(85,506,000)
Commodity Contracts | Not Designated as Hedging Instruments | Total Liabilities
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	$ (216,908,000)	$ (198,353,000)