Derivative Accounting (Details 3) (USD $)	9 Months Ended
	Sep. 30, 2012 item	Dec. 31, 2011
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total Assets	$ 64,706,000	$ 79,586,000
Total Liabilities	(137,507,000)	(136,463,000)
Current assets	26,257,000	30,264,000
Investments and Other Assets	38,449,000	49,322,000
Current Liabilities	(56,263,000)	(53,968,000)
Deferred Credits and Other	(81,244,000)	(82,495,000)
Total Derivative Instruments	(72,801,000)	(56,877,000)
Credit Risk and Credit-Related Contingent Features
Risk management assets	64,706,000	79,586,000
Commodity Contracts
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total Assets	65,000,000
Margin and Collateral Provided to Counterparties	60,833,000	174,626,000
Collateral provided from counterparties	(13,145,000)	(11,145,000)
Other	591,000	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	82.00%
Risk management assets	65,000,000
Aggregate Fair Value of Derivative Instruments in a Net Liability Position	226,000,000
Cash Collateral Posted	60,000,000
Additional Cash Collateral in the Event Credit-Risk Related Contingent Features were Fully Triggered	104,000,000
Additional collateral to counterparties for energy related non-derivative instrument contracts	183,000,000
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	320,000	1,630,000
Other	(31,228,000)	(54,815,000)
Current assets	26,257,000	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	(9,419,000)	(12,755,000)
Investments and Other Assets	38,449,000	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	320,000	1,630,000
Other	(40,647,000)	(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	43,480,000	107,228,000
Collateral provided from counterparties	(13,145,000)	(11,145,000)
Other	31,819,000	56,172,000
Current Liabilities	(56,263,000)	(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	17,033,000	65,768,000
Other	9,419,000	12,754,000
Deferred Credits and Other	(81,244,000)	(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	60,513,000	172,996,000
Collateral provided from counterparties	(13,145,000)	(11,145,000)
Other	41,238,000	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	(5,615,000)	(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		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		3,804,000
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		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	(1,092,000)	(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	(4,523,000)	(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	(5,615,000)	(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	(115,465,000)	(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	57,165,000	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	47,868,000	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	105,033,000	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	(117,325,000)	(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	(103,173,000)	(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	$ (220,498,000)	$ (216,908,000)