Derivative Accounting (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total Assets	$ 61,590,000	$ 79,586,000
Total Liabilities	(159,005,000)	(136,463,000)
Current assets	25,699,000	30,264,000
Investments and Other Assets	35,891,000	49,322,000
Current Liabilities	(73,741,000)	(53,968,000)
Deferred Credits and Other	(85,264,000)	(82,495,000)
Total Derivative Instruments	(97,415,000)	(56,877,000)
Credit Risk and Credit-Related Contingent Features
Risk management assets	61,590,000	79,586,000
Commodity Contracts
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Total Assets	62,000,000
Margin and Collateral Provided to Counterparties	49,361,000	174,626,000
Collateral provided from counterparties	(25,463,000)	(11,145,000)
Other	939,000	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	86.00%
Risk management assets	62,000,000
Aggregate Fair Value of Derivative Instruments in a Net Liability Position	206,000,000
Cash Collateral Posted	49,000,000
Additional Cash Collateral in the Event Credit-Risk Related Contingent Features were Fully Triggered	120,000,000
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	61,000	1,630,000
Other	(16,857,000)	(54,815,000)
Commodity Contracts | Investments and Other Assets
Fair value of derivative instruments, margin account and cash collateral reported on a gross basis
Other	(5,672,000)	(12,755,000)
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	61,000	1,630,000
Other	(22,529,000)	(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	39,249,000	107,228,000
Collateral provided from counterparties	(25,463,000)	(11,145,000)
Other	17,797,000	56,172,000
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	10,051,000	65,768,000
Other	5,671,000	12,754,000
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	49,300,000	172,996,000
Collateral provided from counterparties	(25,463,000)	(11,145,000)
Other	23,468,000	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	(5,479,000)	(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	(1,147,000)	(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	(4,332,000)	(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	(5,479,000)	(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	(116,773,000)	(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	42,495,000	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	41,563,000	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	84,058,000	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	(104,177,000)	(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	(96,654,000)	(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	$ (200,831,000)	$ (216,908,000)