Derivative Commodity Instruments - Commodity Contracts by Business Segment (Details) (USD $)	Sep. 30, 2012 counterparty		Dec. 31, 2011
Derivative [Line Items]
Number of Active Counterparties with Whom Company Holds Net Gain Positions	11
Number of Large Counterparties with Whom Company Holds Net Gain Positions	5
Number of Counterparties with Whom Company Holds Net Gain Positions	3
Derivative, Fair Value, Net	$ 38,420,000		$ (88,820,000)
Deferred tax assets (liabilities), net	(22,000,000)		5,700,000
Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	53,410,000		(28,946,000)
Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	(14,990,000)		(59,874,000)
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	127,535,000		149,618,000
Derivative Liability, Fair Value, Net	(71,261,000)		(141,971,000)
Derivative, Fair Value, Net	56,274,000		7,647,000
Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	127,535,000		149,618,000
Derivative Liability, Fair Value, Net	(71,261,000)		(141,971,000)
Derivative, Fair Value, Net	56,274,000		7,647,000
Designated as Hedging Instrument [Member] | Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Derivative Liability, Fair Value, Net	0		0
Derivative, Fair Value, Net	0		0
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,928,000		(12,255,000)
Derivative Liability, Fair Value, Gross Liability	(20,782,000)		(84,212,000)
Derivative, Fair Value, Net	(17,854,000)		(96,467,000)
Not Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,928,000		(12,255,000)
Derivative Liability, Fair Value, Gross Liability	(5,792,000)		(24,338,000)
Derivative, Fair Value, Net	(2,864,000)		(36,593,000)
Not Designated as Hedging Instrument [Member] | Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Derivative Liability, Fair Value, Gross Liability	(14,990,000)		(59,874,000)
Derivative, Fair Value, Net	(14,990,000)		(59,874,000)
Accounts Receivable [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	76,354,000		73,636,000
Derivative Liability, Fair Value, Gross Asset	(33,476,000)		(48,174,000)
Accounts Receivable [Member] | Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	76,354,000		73,636,000
Derivative Liability, Fair Value, Gross Asset	(33,476,000)	[1]	(48,174,000)	[1]
Accounts Receivable [Member] | Designated as Hedging Instrument [Member] | Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Derivative Liability, Fair Value, Gross Asset	0		0
Accounts Receivable [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	(6,603,000)		(3,670,000)
Accounts Receivable [Member] | Not Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	(6,603,000)	[1]	(3,670,000)	[1]
Accounts Receivable [Member] | Not Designated as Hedging Instrument [Member] | Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Other Assets [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	51,181,000		75,982,000
Derivative Liability, Fair Value, Gross Asset	(17,575,000)		(36,341,000)
Other Assets [Member] | Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	51,181,000		75,982,000
Derivative Liability, Fair Value, Gross Asset	(17,575,000)	[1]	(36,341,000)	[1]
Other Assets [Member] | Designated as Hedging Instrument [Member] | Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Derivative Liability, Fair Value, Gross Asset	0		0
Other Assets [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	9,531,000		(8,585,000)
Other Assets [Member] | Not Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	9,531,000		(8,585,000)	[1]
Other Assets [Member] | Not Designated as Hedging Instrument [Member] | Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Accounts Payable [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(12,471,000)		(37,070,000)
Accounts Payable [Member] | Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(12,471,000)		(37,070,000)
Accounts Payable [Member] | Designated as Hedging Instrument [Member] | Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		0
Accounts Payable [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(19,466,000)		(70,220,000)
Accounts Payable [Member] | Not Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(4,476,000)		(13,416,000)
Accounts Payable [Member] | Not Designated as Hedging Instrument [Member] | Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(14,990,000)		(56,804,000)
Other Liabilities [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(7,739,000)		(20,386,000)
Other Liabilities [Member] | Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(7,739,000)		(20,386,000)
Other Liabilities [Member] | Designated as Hedging Instrument [Member] | Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		0
Other Liabilities [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(1,316,000)		(13,992,000)
Other Liabilities [Member] | Not Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(1,316,000)		(10,922,000)
Other Liabilities [Member] | Not Designated as Hedging Instrument [Member] | Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		(3,070,000)
Cash Flow Hedging [Member] | Macquarie Bank Limited [Member]
Derivative [Line Items]
Derivative Assets	16,800,000
Cash Flow Hedging [Member] | J Aron and Company [Member]
Derivative [Line Items]
Derivative Assets	9,700,000
Cash Flow Hedging [Member] | Citibank, N.A. [Member]
Derivative [Line Items]
Derivative Assets	7,700,000
Cash Flow Hedging [Member] | Shell Energy North America US, L.P. [Member]
Derivative [Line Items]
Derivative Assets	8,500,000
Cash Flow Hedging [Member] | BP Corporation North America Inc. [Member]
Derivative [Line Items]
Derivative Assets	9,600,000
Cash Flow Hedging [Member] | Morgan Stanley Capital Group, Inc. [Member]
Derivative [Line Items]
Derivative Liabilities	$ 24,300,000

[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.