Derivative Accounting (Details 3) (USD $)	Mar. 31, 2012	Dec. 31, 2011
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	$ 87,741,000	$ 79,586,000
Liabilities	(153,375,000)	(136,463,000)
Current assets	34,617,000	30,264,000
Investments and Other Assets	53,124,000	49,322,000
Current Liabilities	(89,207,000)	(53,968,000)
Deferred Credits and Other	(64,168,000)	(82,495,000)
Derivative Instruments	(65,634,000)	(56,877,000)
Credit Risk and Credit-Related Contingent Features
Risk management assets	87,741,000	79,586,000
Commodity Contracts
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Assets	88,000,000
Margin and Collateral Provided to Counterparties	209,432,000	174,626,000
Collateral provided from counterparties	(11,145,000)	(11,145,000)
Other	1,331,000	1,356,000
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	83.00%
Risk management assets	88,000,000
Aggregate Fair Value of Derivative Instruments in a Liability Position	380,000,000
Cash Collateral Posted	180,000,000
Additional Cash Collateral in the Event Credit-Risk Related Contingent Features were Fully Triggered	169,000,000
Additional collateral to counterparties for energy related non-derivative instrument contracts	184,000,000
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	3,486,000	1,630,000
Other	(55,699,000)	(54,815,000)
Current assets	34,617,000	30,264,000
Commodity Contracts | Investments and Other Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Other	(7,892,000)	(12,755,000)
Investments and Other Assets	53,124,000	49,322,000
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	3,486,000	1,630,000
Other	(63,591,000)	(67,570,000)
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	100,228,000	107,228,000
Collateral provided from counterparties	(11,145,000)	(11,145,000)
Other	57,053,000	56,172,000
Current Liabilities	(89,207,000)	(53,968,000)
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	105,718,000	65,768,000
Other	7,869,000	12,754,000
Deferred Credits and Other	(64,168,000)	(82,495,000)
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	205,946,000	172,996,000
Collateral provided from counterparties	(11,145,000)	(11,145,000)
Other	64,922,000	68,926,000
Commodity Contracts | Designated as Hedging Instruments
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(174,001,000)	(139,241,000)
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	6,466,000	7,287,000
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	3,377,000	3,804,000
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	9,843,000	11,091,000
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	(103,592,000)	(82,195,000)
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	(80,252,000)	(68,137,000)
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	(183,844,000)	(150,332,000)
Commodity Contracts | Not Designated as Hedging Instruments
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total derivatives	(91,251,000)	(82,473,000)
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	80,364,000	76,162,000
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	57,639,000	58,273,000
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	138,003,000	134,435,000
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	(131,751,000)	(124,028,000)
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	(97,503,000)	(92,880,000)
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	$ (229,254,000)	$ (216,908,000)