Derivative Commodity Instruments (Details) - Fair Value Derivatives (USD $) In Thousands	Sep. 30, 2011		Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 116,311		$ (130,908)
Oil and Gas Operations
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	173,498		(70,541)
Oil and Gas Operations | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	156,300		89,023
Derivative Liability, Fair Value, Net	(35,989)		(159,454)
Derivative, Fair Value, Net	120,311		(70,431)
Oil and Gas Operations | Designated as Hedging Instrument | Accounts receivable
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	88,839		85,867
Derivative Liability, Fair Value, Gross Asset	(25,642)	[1]	(25,315)	[1]
Oil and Gas Operations | Designated as Hedging Instrument | Accounts Payable
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(1,298)		(50,508)
Oil and Gas Operations | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	67,461	[1]	3,156	[1]
Oil and Gas Operations | Designated as Hedging Instrument | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(9,049)		(83,631)
Oil and Gas Operations | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	53,227
Derivative Liability, Fair Value, Gross Liability	(40)		(110)
Derivative, Fair Value, Net	53,187		(110)
Oil and Gas Operations | Not Designated as Hedging Instrument | Accounts receivable
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	10,745
Oil and Gas Operations | Not Designated as Hedging Instrument | Accounts Payable
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(40)		(110)
Oil and Gas Operations | Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	42,482	[1]
Oil and Gas Operations | Not Designated as Hedging Instrument | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		0
Natural Gas Distribution
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(57,187)		(60,367)
Natural Gas Distribution | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Derivative Liability, Fair Value, Net	0		0
Derivative, Fair Value, Net	0		0
Natural Gas Distribution | Designated as Hedging Instrument | Accounts receivable
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Derivative Liability, Fair Value, Gross Asset	0		0
Natural Gas Distribution | Designated as Hedging Instrument | Accounts Payable
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		0
Natural Gas Distribution | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Natural Gas Distribution | Designated as Hedging Instrument | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		0
Natural Gas Distribution | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0
Derivative Liability, Fair Value, Gross Liability	(57,187)		(60,367)
Derivative, Fair Value, Net	(57,187)		(60,367)
Natural Gas Distribution | Not Designated as Hedging Instrument | Accounts receivable
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0
Natural Gas Distribution | Not Designated as Hedging Instrument | Accounts Payable
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(44,137)		(27,906)
Natural Gas Distribution | Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0
Natural Gas Distribution | Not Designated as Hedging Instrument | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(13,050)		(32,461)
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	156,300		89,023
Derivative Liability, Fair Value, Net	(35,989)		(159,454)
Derivative, Fair Value, Net	120,311		(70,431)
Designated as Hedging Instrument | Accounts receivable
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	88,839		85,867
Derivative Liability, Fair Value, Gross Asset	(25,642)		(25,315)
Designated as Hedging Instrument | Accounts Payable
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(1,298)		(50,508)
Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	67,461		3,156
Designated as Hedging Instrument | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(9,049)		(83,631)
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	53,227
Derivative Liability, Fair Value, Gross Liability	(57,227)		(60,477)
Derivative, Fair Value, Net	(4,000)		(60,477)
Not Designated as Hedging Instrument | Accounts receivable
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	10,745
Not Designated as Hedging Instrument | Accounts Payable
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(44,177)		(28,016)
Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	42,482
Not Designated as Hedging Instrument | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ (13,050)		$ (32,461)

[1] Amounts classified in accordance with accounting guidance which permits offsetting fair value amounts recognized for multiple derivative instruments executed with the same counterparty under a master netting arrangement.