Financial Instruments and Risk Management (Risk Management) (Details) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Derivative, Fair Value, Net [Abstract]
Derivative, Fair Value, Net	$ (88,820,000)	$ (130,908,000)
Deferred Tax Assets (Liabilities), Net [Abstract]
Deferred tax assets (liabilities), net	(5,700,000)	(26,800,000)
Designated as Hedging Instrument
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	149,618,000	89,023,000
Derivative Liability, Fair Value, Net	(141,971,000)	(159,454,000)
Derivative, Fair Value, Net	7,647,000	(70,431,000)
Designated as Hedging Instrument | Accounts receivable
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	73,636,000	85,867,000
Derivative Liability, Fair Value, Gross Asset	(48,174,000)	(25,315,000)
Designated as Hedging Instrument | Other Assets
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	75,982,000	3,156,000
Derivative Liability, Fair Value, Gross Asset	(36,341,000)
Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(37,070,000)	(50,508,000)
Designated as Hedging Instrument | Other Liabilities
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(20,386,000)	(83,631,000)
Not Designated as Hedging Instrument
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	(12,255,000)
Derivative Liability, Fair Value, Gross Liability	(84,212,000)	(60,477,000)
Derivative, Fair Value, Net	(96,467,000)	(60,477,000)
Not Designated as Hedging Instrument | Accounts receivable
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	(3,670,000)
Not Designated as Hedging Instrument | Other Assets
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	(8,585,000)
Not Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(70,220,000)	(28,016,000)
Not Designated as Hedging Instrument | Other Liabilities
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(13,992,000)	(32,461,000)
Oil and Gas Operations
Derivative, Fair Value, Net [Abstract]
Derivative, Fair Value, Net	(28,946,000)	(70,541,000)
Oil and Gas Operations | Designated as Hedging Instrument
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	149,618,000	89,023,000
Derivative Liability, Fair Value, Net	(141,971,000)	(159,454,000)
Derivative, Fair Value, Net	7,647,000	(70,431,000)
Oil and Gas Operations | Designated as Hedging Instrument | Accounts receivable
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	73,636,000	85,867,000
Derivative Liability, Fair Value, Gross Asset	(48,174,000)	(25,315,000)	[1]
Oil and Gas Operations | Designated as Hedging Instrument | Other Assets
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	75,982,000	3,156,000	[1]
Derivative Liability, Fair Value, Gross Asset	(36,341,000)
Oil and Gas Operations | Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(37,070,000)	(50,508,000)
Oil and Gas Operations | Designated as Hedging Instrument | Other Liabilities
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(20,386,000)	(83,631,000)
Oil and Gas Operations | Not Designated as Hedging Instrument
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	(12,255,000)
Derivative Liability, Fair Value, Gross Liability	(24,338,000)	(110,000)
Derivative, Fair Value, Net	(36,593,000)	(110,000)
Oil and Gas Operations | Not Designated as Hedging Instrument | Accounts receivable
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	(3,670,000)
Oil and Gas Operations | Not Designated as Hedging Instrument | Other Assets
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	(8,585,000)
Oil and Gas Operations | Not Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(13,416,000)	(110,000)
Oil and Gas Operations | Not Designated as Hedging Instrument | Other Liabilities
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(10,922,000)	0
Natural Gas Distribution
Derivative, Fair Value, Net [Abstract]
Derivative, Fair Value, Net	(59,874,000)	(60,367,000)
Natural Gas Distribution | Designated as Hedging Instrument
Derivative, Fair Value, Net [Abstract]
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
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Asset	0	0
Natural Gas Distribution | Designated as Hedging Instrument | Other Assets
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Gross Asset	0
Natural Gas Distribution | Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	0
Natural Gas Distribution | Designated as Hedging Instrument | Other Liabilities
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	0
Natural Gas Distribution | Not Designated as Hedging Instrument
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	0
Derivative Liability, Fair Value, Gross Liability	(59,874,000)	(60,367,000)
Derivative, Fair Value, Net	(59,874,000)	(60,367,000)
Natural Gas Distribution | Not Designated as Hedging Instrument | Accounts receivable
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	0
Natural Gas Distribution | Not Designated as Hedging Instrument | Other Assets
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	0
Natural Gas Distribution | Not Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(56,804,000)	(27,906,000)
Natural Gas Distribution | Not Designated as Hedging Instrument | Other Liabilities
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(3,070,000)	(32,461,000)
Cash Flow Hedges | Morgan Stanley Capital Group, Inc.
Derivatives, Fair Value [Line Items]
Derivative, Net Loss Position	55,300,000
Cash Flow Hedges | Macquarie Bank Limited
Derivatives, Fair Value [Line Items]
Derivative, Net Gain Position	13,800,000
Cash Flow Hedges | Shell Energy North America US, L.P.
Derivatives, Fair Value [Line Items]
Derivative, Net Gain Position	10,300,000
Cash Flow Hedges | Barclays Bank PLC
Derivatives, Fair Value [Line Items]
Derivative, Net Gain Position	$ 9,900,000

[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.