Derivative Commodity Instruments - Commodity Contracts by Business Segment (Details) (USD $)	Mar. 31, 2013 bbl counterparty		Dec. 31, 2012
Derivative [Line Items]
Number of active counterparties with whom company holds net loss positions	6
Number of large counterparties with whom company holds net loss positions	2
Derivative liabilities	$ 41,148,000		$ 12,193,000
Number of active counterparties with whom company holds net gain positions	7
Number of large counterparties with whom company holds net gain positions	2
Derivative assets	36,889,000		105,369,000
Barrels dedesignated at March 31, 2013	5,078,000
Gross Amounts Recognized	79,728,000		149,504,000
Derivative liability	(83,987,000)		(56,328,000)
Total derivatives	(4,259,000)		93,176,000
Deferred tax assets (liabilities), net	(5,400,000)		(28,400,000)
Oil and gas operations
Derivative [Line Items]
Total derivatives	(4,259,000)		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	77,198,000		125,468,000
Total derivative liabilities	(62,097,000)		(52,862,000)
Total derivatives	15,101,000		72,606,000
Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	77,198,000		125,468,000
Total derivative liabilities	(62,097,000)		(52,862,000)
Total derivatives	15,101,000		72,606,000
Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Gross Amounts Recognized	0		0
Total derivative liabilities	0		0
Total derivatives	0		0
Not Designated as Hedging Instrument
Derivative [Line Items]
Gross Amounts Recognized	2,530,000		24,037,000
Derivative liability	(21,890,000)		(3,467,000)
Total derivatives	(19,360,000)		20,570,000
Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	2,530,000		24,037,000
Derivative liability	(21,890,000)		(874,000)
Total derivatives	(19,360,000)		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	0		(2,593,000)
Accounts receivable | Designated as Hedging Instrument
Derivative [Line Items]
Gross Amounts Recognized	51,277,000		87,514,000
Derivative liability	(36,245,000)		(37,326,000)
Accounts receivable | Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	51,277,000		87,514,000
Derivative liability	(36,245,000)	[1]	(37,326,000)	[1]
Accounts receivable | Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Gross Amounts Recognized	0		0
Derivative liability	0		0
Accounts receivable | Not Designated as Hedging Instrument
Derivative [Line Items]
Gross Amounts Recognized	(4,342,000)		14,604,000
Accounts receivable | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	(4,342,000)	[1]	14,604,000	[1]
Accounts receivable | Not Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Gross Amounts Recognized	0		0
Long-term asset derivative instruments | Designated as Hedging Instrument
Derivative [Line Items]
Gross Amounts Recognized	25,921,000		37,954,000
Derivative liability	(6,594,000)		(6,810,000)
Long-term asset derivative instruments | Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	25,921,000		37,954,000
Derivative liability	(6,594,000)	[1]	(6,810,000)	[1]
Long-term asset derivative instruments | Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Gross Amounts Recognized	0		0
Derivative liability	0		0
Long-term asset derivative instruments | Not Designated as Hedging Instrument
Derivative [Line Items]
Gross Amounts Recognized	6,872,000		9,433,000
Long-term asset derivative instruments | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Gross Amounts Recognized	6,872,000		9,433,000	[1]
Long-term asset derivative instruments | Not Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Gross Amounts Recognized	0		0
Accounts payable | Designated as Hedging Instrument
Derivative [Line Items]
Derivative liability	(11,983,000)
Accounts payable | Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Derivative liability	(11,983,000)
Accounts payable | Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Derivative liability	0
Accounts payable | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative liability	(20,816,000)		(2,593,000)
Accounts payable | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Derivative liability	(20,816,000)		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	(7,275,000)		(8,726,000)
Long-term liability derivative instruments | Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Derivative liability	(7,275,000)		(8,726,000)
Long-term liability derivative instruments | Designated as Hedging Instrument | Natural gas distribution
Derivative [Line Items]
Derivative liability	0		0
Long-term liability derivative instruments | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative liability	(1,074,000)		(874,000)
Long-term liability derivative instruments | Not Designated as Hedging Instrument | Oil and gas operations
Derivative [Line Items]
Derivative liability	(1,074,000)		(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	10,300,000
Cash Flow Hedging | J Aron and Company
Derivative [Line Items]
Derivative assets	5,300,000
Cash Flow Hedging | Citibank, N.A.
Derivative [Line Items]
Derivative liabilities	5,100,000
Cash Flow Hedging | Morgan Stanley Capital Group, Inc.
Derivative [Line Items]
Derivative liabilities	$ 24,700,000

[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.