Derivative Commodity Instruments - Commodity Contracts by Business Segment (Details) (USD $)	Sep. 30, 2013		Jun. 30, 2013 bbl	Mar. 31, 2013 bbl	Dec. 31, 2012
Derivative [Line Items]
Derivative assets	$ 25,079,000				$ 105,369,000
Fair Value as of September 30, 2013	33,307,000				12,193,000
Barrels dedesignated			2,353,000	5,078,000
Gross Amounts Recognized	71,428,000				149,504,000
Derivative liability	(79,656,000)				(56,328,000)
Total derivatives	(8,228,000)				93,176,000
Deferred tax assets (liabilities), net	(10,300,000)				(28,400,000)
Oil and gas operations
Derivative [Line Items]
Total derivatives	(8,228,000)				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	71,428,000				24,037,000
Derivative liability	(79,656,000)				(3,467,000)
Total derivatives					20,570,000
Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	71,428,000				24,037,000
Derivative liability	(79,656,000)				(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	53,082,000				14,604,000
Derivative liability	(40,789,000)
Accounts receivable | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	53,082,000	[1]			14,604,000	[1]
Derivative liability	(40,789,000)	[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	18,346,000				9,433,000
Derivative liability	(5,560,000)
Long-term asset derivative instruments | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	18,346,000				9,433,000	[1]
Derivative liability	(5,560,000)	[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	(31,928,000)				(2,593,000)
Accounts payable | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Derivative liability	(31,928,000)				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	(1,379,000)				(874,000)
Long-term liability derivative instruments | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Derivative liability	(1,379,000)				(874,000)
Long-term liability derivative instruments | Not Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Derivative liability	0				0
Cash Flow Hedging | Macquarie Bank Limited
Derivative [Line Items]
Derivative assets	10,000,000
Cash Flow Hedging | Morgan Stanley Capital Group, Inc.
Derivative [Line Items]
Fair Value as of September 30, 2013	$ 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.