Derivative Commodity Instruments - Commodity Contracts by Business Segment (Details) (USD $)	Mar. 31, 2012		Dec. 31, 2011
Derivative [Line Items]
Derivative, Fair Value, Net	$ (141,763,000)		$ (88,820,000)
Deferred tax assets (liabilities), net	(1,500,000)		5,700,000
Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	(93,878,000)		(28,946,000)
Natural Gas Distribution [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	(47,885,000)		(59,874,000)
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	185,528,000		149,618,000
Derivative Liability, Fair Value, Net	(200,392,000)		(141,971,000)
Derivative, Fair Value, Net	(14,864,000)		7,647,000
Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	185,528,000		149,618,000
Derivative Liability, Fair Value, Net	(200,392,000)		(141,971,000)
Derivative, Fair Value, Net	(14,864,000)		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	(23,159,000)		(12,255,000)
Derivative Liability, Fair Value, Gross Liability	(103,740,000)		(84,212,000)
Derivative, Fair Value, Net	(126,899,000)		(96,467,000)
Not Designated as Hedging Instrument [Member] | Oil and Gas Operations [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	(23,159,000)		(12,255,000)
Derivative Liability, Fair Value, Gross Liability	(55,855,000)		(24,338,000)
Derivative, Fair Value, Net	(79,014,000)		(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	(47,885,000)		(59,874,000)
Derivative, Fair Value, Net	(47,885,000)		(59,874,000)
Accounts Receivable [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	90,764,000		73,636,000
Derivative Liability, Fair Value, Gross Asset	(65,461,000)		(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	90,764,000		73,636,000
Derivative Liability, Fair Value, Gross Asset	(65,461,000)	[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	(3,624,000)		(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	(3,624,000)	[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	94,764,000		75,982,000
Derivative Liability, Fair Value, Gross Asset	(64,621,000)		(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	94,764,000		75,982,000
Derivative Liability, Fair Value, Gross Asset	(64,621,000)	[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
Other Assets [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	(19,535,000)		(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	(19,535,000)	[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	(36,536,000)		(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	(36,536,000)		(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	(76,801,000)		(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	(28,916,000)		(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	(47,885,000)		(56,804,000)
Other Liabilities [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	(33,774,000)		(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	(33,774,000)		(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	(26,939,000)		(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	(26,939,000)		(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] | Counterparty, Net Loss Position [Member]
Derivative [Line Items]
Number of Active Counterparties with Whom Company Holds Net Loss Positions	8
Number of Large Counterparties with Whom Company Holds Net Loss Positions	3
Cash Flow Hedging [Member] | Morgan Stanley Capital Group, Inc. [Member]
Derivative [Line Items]
Derivative, Net Loss Position	73,600,000
Cash Flow Hedging [Member] | J Aron and Company [Member]
Derivative [Line Items]
Derivative, Net Loss Position	13,900,000
Cash Flow Hedging [Member] | Credit Suisse Energy LLC [Member]
Derivative [Line Items]
Derivative, Net Loss Position	11,300,000
Cash Flow Hedging [Member] | Counterparty, Net Gain Position [Member]
Derivative [Line Items]
Number of Counterparties with Whom Company Holds Net Gain Positions	5
Cash Flow Hedging [Member] | Counterparty, Net Gain Position [Member] | Minimum [Member]
Derivative [Line Items]
Derivative Assets	2,000,000
Cash Flow Hedging [Member] | Counterparty, Net Gain Position [Member] | Maximum [Member]
Derivative [Line Items]
Derivative Assets	$ 6,000,000

[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.