Derivative Commodity Instruments (Details) - Fair Value Derivatives (USD $) In Thousands	Jun. 30, 2011		Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ (202,651)		$ (130,908)
Oil and Gas Operations
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(152,835)		(70,541)
Oil and Gas Operations | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	59,609		89,023
Derivative Liability, Fair Value, Net	(212,376)		(159,454)
Derivative, Fair Value, Net	(152,767)		(70,431)
Oil and Gas Operations | Designated as Hedging Instrument | Accounts receivable
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	49,182		85,867
Derivative Liability, Fair Value, Gross Asset	(37,177)	[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	(54,926)		(50,508)
Oil and Gas Operations | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	10,427	[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	(120,273)		(83,631)
Oil and Gas Operations | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(68)		(110)
Derivative, Fair Value, Net	(68)		(110)
Oil and Gas Operations | Not Designated as Hedging Instrument | Accounts Payable
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(68)		(110)
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	(49,816)		(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 Liability, Fair Value, Gross Liability	(49,816)		(60,367)
Derivative, Fair Value, Net	(49,816)		(60,367)
Natural Gas Distribution | Not Designated as Hedging Instrument | Accounts Payable
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(32,159)		(27,906)
Natural Gas Distribution | Not Designated as Hedging Instrument | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(17,657)		(32,461)
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	59,609		89,023
Derivative Liability, Fair Value, Net	(212,376)		(159,454)
Derivative, Fair Value, Net	(152,767)		(70,431)
Designated as Hedging Instrument | Accounts receivable
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	49,182		85,867
Derivative Liability, Fair Value, Gross Asset	(37,177)		(25,315)
Designated as Hedging Instrument | Accounts Payable
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(54,926)		(50,508)
Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	10,427		3,156
Designated as Hedging Instrument | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(120,273)		(83,631)
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(49,884)		(60,477)
Derivative, Fair Value, Net	(49,884)		(60,477)
Not Designated as Hedging Instrument | Accounts Payable
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(32,227)		(28,016)
Not Designated as Hedging Instrument | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ (17,657)		$ (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.