Financial Instruments and Risk Management (Risk Management) (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Derivative, Fair Value, Net [Abstract]
Derivative, Fair Value, Net	$ 93,176,000	$ (88,820,000)
Deferred Tax Assets (Liabilities), Net [Abstract]
Deferred tax assets (liabilities), net	(28,400,000)	(5,700,000)
Designated as Hedging Instrument
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	125,468,000	149,618,000
Derivative Liability, Fair Value, Net	(52,862,000)	(141,971,000)
Derivative, Fair Value, Net	72,606,000	7,647,000
Designated as Hedging Instrument | Accounts receivable
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	87,514,000	73,636,000
Derivative Liability, Fair Value, Gross Asset	(37,326,000)	(48,174,000)
Designated as Hedging Instrument | Other Assets
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	37,954,000	75,982,000
Derivative Liability, Fair Value, Gross Asset	(6,810,000)	(36,341,000)
Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability		(37,070,000)
Designated as Hedging Instrument | Other Liabilities
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(8,726,000)	(20,386,000)
Not Designated as Hedging Instrument
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	24,037,000	(12,255,000)
Derivative Liability, Fair Value, Gross Liability	(3,467,000)	(84,212,000)
Derivative, Fair Value, Net	20,570,000	(96,467,000)
Not Designated as Hedging Instrument | Accounts receivable
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	14,604,000	(3,670,000)
Not Designated as Hedging Instrument | Other Assets
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	9,433,000	(8,585,000)
Not Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(2,593,000)	(70,220,000)
Not Designated as Hedging Instrument | Other Liabilities
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(874,000)	(13,992,000)
Oil and Gas Operations
Derivative, Fair Value, Net [Abstract]
Derivative, Fair Value, Net	95,769,000	(28,946,000)
Oil and Gas Operations | Designated as Hedging Instrument
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	125,468,000	149,618,000
Derivative Liability, Fair Value, Net	(52,862,000)	(141,971,000)
Derivative, Fair Value, Net	72,606,000	7,647,000
Oil and Gas Operations | Designated as Hedging Instrument | Accounts receivable
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	87,514,000	73,636,000
Derivative Liability, Fair Value, Gross Asset	(37,326,000)	(48,174,000)	[1]
Oil and Gas Operations | Designated as Hedging Instrument | Other Assets
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	37,954,000	75,982,000
Derivative Liability, Fair Value, Gross Asset	(6,810,000)	(36,341,000)	[1]
Oil and Gas Operations | Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability		(37,070,000)
Oil and Gas Operations | Designated as Hedging Instrument | Other Liabilities
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(8,726,000)	(20,386,000)
Oil and Gas Operations | Not Designated as Hedging Instrument
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	24,037,000	(12,255,000)
Derivative Liability, Fair Value, Gross Liability	(874,000)	(24,338,000)
Derivative, Fair Value, Net	23,163,000	(36,593,000)
Oil and Gas Operations | Not Designated as Hedging Instrument | Accounts receivable
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	14,604,000	(3,670,000)	[1]
Oil and Gas Operations | Not Designated as Hedging Instrument | Other Assets
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	9,433,000	(8,585,000)	[1]
Oil and Gas Operations | Not Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	(13,416,000)
Oil and Gas Operations | Not Designated as Hedging Instrument | Other Liabilities
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(874,000)	(10,922,000)
Natural Gas Distribution
Derivative, Fair Value, Net [Abstract]
Derivative, Fair Value, Net	(2,593,000)	(59,874,000)
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	0
Natural Gas Distribution | Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability		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	0
Derivative Liability, Fair Value, Gross Liability	(2,593,000)	(59,874,000)
Derivative, Fair Value, Net	(2,593,000)	(59,874,000)
Natural Gas Distribution | Not Designated as Hedging Instrument | Accounts receivable
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	0	0
Natural Gas Distribution | Not Designated as Hedging Instrument | Other Assets
Derivative, Fair Value, Net [Abstract]
Derivative Asset, Fair Value, Gross Asset	0	0
Natural Gas Distribution | Not Designated as Hedging Instrument | Accounts Payable
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	(2,593,000)	(56,804,000)
Natural Gas Distribution | Not Designated as Hedging Instrument | Other Liabilities
Derivative, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Gross Liability	0	(3,070,000)
Cash Flow Hedges | Macquarie Bank Limited
Derivatives, Fair Value [Line Items]
Derivative, Net Loss Position	20,000,000
Cash Flow Hedges | J Aron & Company
Derivatives, Fair Value [Line Items]
Derivative, Net Gain Position	16,600,000
Cash Flow Hedges | BP Corporation North America Inc
Derivatives, Fair Value [Line Items]
Derivative, Net Gain Position	13,600,000
Cash Flow Hedges | Shell Energy North America US, L.P.
Derivatives, Fair Value [Line Items]
Derivative, Net Gain Position	$ 10,300,000

[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.