Derivative Commodity Instruments - Commodity Contracts by Business Segment (Details) (USD $)	Jun. 30, 2013 bbl counterparty		Mar. 31, 2013 bbl	Dec. 31, 2012
Derivative [Line Items]
Number of Active Counterparties with Whom Company Holds Net Gain Positions	12
Number of Large Counterparties with Whom Company Holds Net Gain Positions	3
Derivative assets	$ 85,087,000			$ 105,369,000
Fair Value as of June 30, 2013	12,506,000			12,193,000
Barrels dedesignated at March 31, 2013	2,353,000		5,078,000
Gross Amounts Recognized	114,784,000			149,504,000
Derivative liability	(42,203,000)			(56,328,000)
Total derivatives	72,581,000			93,176,000
Deferred tax assets (liabilities), net	(13,400,000)			(28,400,000)
Oil and gas operations
Derivative [Line Items]
Total derivatives	72,581,000			95,769,000
Natural gas distribution
Derivative [Line Items]
Total derivatives	0			(2,593,000)
Designated as Hedging Instrument
Derivative [Line Items]
Gross Amounts Recognized				125,468,000
Total derivative liabilities				(52,862,000)
Total derivatives				72,606,000
Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized				125,468,000
Total derivative liabilities				(52,862,000)
Total derivatives				72,606,000
Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Gross Amounts Recognized				0
Total derivative liabilities				0
Total derivatives				0
Not Designated as Hedging Instrument
Derivative [Line Items]
Gross Amounts Recognized	114,784,000			24,037,000
Derivative liability	(42,203,000)			(3,467,000)
Total derivatives				20,570,000
Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	114,784,000			24,037,000
Derivative liability	(42,203,000)			(874,000)
Total derivatives				23,163,000
Not Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Gross Amounts Recognized	0			0
Derivative liability	0			(2,593,000)
Total derivatives				(2,593,000)
Accounts receivable | Designated as Hedging Instrument
Derivative [Line Items]
Gross Amounts Recognized				87,514,000
Derivative liability				(37,326,000)
Accounts receivable | Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized				87,514,000
Derivative liability				(37,326,000)	[1]
Accounts receivable | Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Gross Amounts Recognized				0
Derivative liability				0
Accounts receivable | Not Designated as Hedging Instrument
Derivative [Line Items]
Gross Amounts Recognized	72,679,000			14,604,000
Derivative liability	(26,598,000)
Accounts receivable | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	72,679,000	[1]		14,604,000	[1]
Derivative liability	(26,598,000)	[1]
Accounts receivable | Not Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Gross Amounts Recognized	0			0
Derivative liability	0
Long-term asset derivative instruments | Designated as Hedging Instrument
Derivative [Line Items]
Gross Amounts Recognized				37,954,000
Derivative liability				(6,810,000)
Long-term asset derivative instruments | Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized				37,954,000
Derivative liability				(6,810,000)	[1]
Long-term asset derivative instruments | Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Gross Amounts Recognized				0
Derivative liability				0
Long-term asset derivative instruments | Not Designated as Hedging Instrument
Derivative [Line Items]
Gross Amounts Recognized	42,105,000			9,433,000
Derivative liability	(3,099,000)
Long-term asset derivative instruments | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	42,105,000			9,433,000	[1]
Derivative liability	(3,099,000)	[1]
Long-term asset derivative instruments | Not Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Gross Amounts Recognized	0			0
Derivative liability	0
Accounts payable | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative liability	(12,506,000)			(2,593,000)
Accounts payable | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Derivative liability	(12,506,000)			0
Accounts payable | Not Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Derivative liability	0			(2,593,000)
Long-term liability derivative instruments | Designated as Hedging Instrument
Derivative [Line Items]
Derivative liability				(8,726,000)
Long-term liability derivative instruments | Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Derivative liability				(8,726,000)
Long-term liability derivative instruments | Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Derivative liability				0
Long-term liability derivative instruments | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative liability				(874,000)
Long-term liability derivative instruments | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Derivative liability				(874,000)
Long-term liability derivative instruments | Not Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Derivative liability				0
Cash Flow Hedging | Macquarie Bank Limited
Derivative [Line Items]
Derivative assets	16,500,000
Cash Flow Hedging | BP Corporation North America Inc
Derivative [Line Items]
Derivative assets	11,700,000
Cash Flow Hedging | J Aron & Company
Derivative [Line Items]
Derivative assets	11,500,000
Cash Flow Hedging | Morgan Stanley Capital Group, Inc.
Derivative [Line Items]
Fair Value as of June 30, 2013	$ 11,400,000

[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.