Derivative Commodity Instruments - Commodity Contracts by Business Segment (Details) (USD $)	Jun. 30, 2012		Dec. 31, 2011
Derivative [Line Items]
Derivative, Fair Value, Net	$ 173,711,000		$ (88,820,000)
Deferred tax assets (liabilities), net	(57,500,000)		5,700,000
Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	204,285,000		(28,946,000)
Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	(30,574,000)		(59,874,000)
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	187,704,000		149,618,000
Derivative Liability, Fair Value, Net	(29,408,000)		(141,971,000)
Derivative, Fair Value, Net	158,296,000		7,647,000
Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	187,704,000		149,618,000
Derivative Liability, Fair Value, Net	(29,408,000)		(141,971,000)
Derivative, Fair Value, Net	158,296,000		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	44,155,000		(12,255,000)
Derivative Liability, Fair Value, Gross Liability	(28,740,000)		(84,212,000)
Derivative, Fair Value, Net	15,415,000		(96,467,000)
Not Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	44,155,000		(12,255,000)
Derivative Liability, Fair Value, Gross Liability	1,834,000		(24,338,000)
Derivative, Fair Value, Net	45,989,000		(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	(30,574,000)		(59,874,000)
Derivative, Fair Value, Net	(30,574,000)		(59,874,000)
Accounts Receivable [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	102,499,000		73,636,000
Derivative Liability, Fair Value, Gross Asset	(13,592,000)		(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	102,499,000		73,636,000
Derivative Liability, Fair Value, Gross Asset	(13,592,000)	[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	14,256,000		(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	14,256,000	[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	85,205,000		75,982,000
Derivative Liability, Fair Value, Gross Asset	(11,831,000)		(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	85,205,000		75,982,000
Derivative Liability, Fair Value, Gross Asset	(11,831,000)	[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	29,899,000		(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	29,899,000	[1]	(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	(3,940,000)		(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	(3,940,000)		(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	(28,740,000)		(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	1,834,000		(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	(30,574,000)		(56,804,000)
Other Liabilities [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(45,000)		(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	(45,000)		(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			(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			(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			(3,070,000)
Cash Flow Hedging [Member] | Macquarie Bank Limited [Member]
Derivative [Line Items]
Derivative Assets	33,100,000
Cash Flow Hedging [Member] | J Aron and Company [Member]
Derivative [Line Items]
Derivative Assets	31,200,000
Cash Flow Hedging [Member] | Citibank, N.A. [Member]
Derivative [Line Items]
Derivative Assets	23,900,000
Cash Flow Hedging [Member] | Shell Energy North America US, L.P. [Member]
Derivative [Line Items]
Derivative Assets	22,900,000
Cash Flow Hedging [Member] | BP Corporation North America Inc. [Member]
Derivative [Line Items]
Derivative Assets	$ 19,800,000

[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.