Acquisition and Dispositions of Oil and Gas Properties (Details) (USD $)	3 Months Ended								12 Months Ended				0 Months Ended			12 Months Ended	0 Months Ended					0 Months Ended										3 Months Ended								12 Months Ended			1 Months Ended
	Dec. 31, 2013 MMBoe	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012 MMBoe	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 MMBoe	Dec. 31, 2012 MMBoe	Dec. 31, 2011 MMBoe	Dec. 31, 2010 MMBoe	Feb. 21, 2012 Feb 21, 2012, Reeves County acre well	May 01, 2012 Feb 21, 2012, Reeves County Contingent Sales Agreement acre	Feb. 14, 2012 Feb 14, 2012, Permian Basin MMBoe	Dec. 31, 2012 Feb 14, 2012, Permian Basin	Feb. 14, 2012 Feb 14, 2012, Permian Basin Oil Reserves	Feb. 14, 2012 Feb 14, 2012, Permian Basin Natural Gas Liquids Reserves	Feb. 14, 2012 Feb 14, 2012, Permian Basin Natural Gas Reserves	Dec. 31, 2013 December 2012, Permian Basin	Dec. 31, 2013 Unproved Leasehold Properties	Dec. 27, 2011 Dec 27, 2011, Permian Basin MMBoe	Dec. 27, 2011 Dec 27, 2011, Permian Basin Oil Reserves	Dec. 27, 2011 Dec 27, 2011, Permian Basin Natural Gas Liquids Reserves	Dec. 27, 2011 Dec 27, 2011, Permian Basin Natural Gas Reserves	Nov. 16, 2011 Nov 16, 2011, Permian Basin MMBoe	Nov. 16, 2011 Nov 16, 2011, Permian Basin Oil Reserves	Nov. 16, 2011 Nov 16, 2011, Permian Basin Natural Gas Liquids Reserves	Nov. 16, 2011 Nov 16, 2011, Permian Basin Natural Gas Reserves	Jul. 31, 2011 July 2011, Permian Basin	Apr. 30, 2011 April 2011, Permian Basin acre	Dec. 31, 2013 Alabama Gas Corporation	Sep. 30, 2013 Alabama Gas Corporation	Jun. 30, 2013 Alabama Gas Corporation	Mar. 31, 2013 Alabama Gas Corporation	Dec. 31, 2012 Alabama Gas Corporation	Sep. 30, 2012 Alabama Gas Corporation	Jun. 30, 2012 Alabama Gas Corporation	Mar. 31, 2012 Alabama Gas Corporation	Dec. 31, 2013 Alabama Gas Corporation	Dec. 31, 2012 Alabama Gas Corporation	Dec. 31, 2011 Alabama Gas Corporation	Aug. 31, 2013 Birmingham Metro Operations Center Alabama Gas Corporation	Sep. 30, 2013 Birmingham Metro Operations Center Alabama Gas Corporation
Significant Acquisitions and Disposals [Line Items]
Gain of disposition of Metro Operations Center																																											$ 10,900,000
Sales price																																												13,800,000
Lease back period after sale																																											20 months
Acquisitions, net of cash acquired									(31,331,000)	(139,563,000)	(310,193,000)
Significant Acquisitions and Disposals, Percentage of Undivided Interest, Sale													50.00%	50.00%
Acquisition Oil and Gas Properties, Number of Wells													3
Cash received from sale of oil properties													18,000,000
Net acres													4,829	51,720																	11,000
Consideration - Purchase Price															67,615,000					18,700,000	18,000,000	60,017,000				161,967,000				20,000,000	37,000,000
Proved reserves acquired	347.8				346.4				347.8	346.4	343.1	302.9			8.2							3.4				13.6
Percentage of undeveloped portion of proved reserves									45.00%	59.00%	69.00%				81.00%							77.00%				76.00%
Acquired proved reserves by type of reserve, percentage																	64.00%	22.00%	14.00%				61.00%	24.00%	15.00%		59.00%	25.00%	16.00%
Oil and Gas Property Acquisition, Pro Forma Information [Abstract]
Operating revenue	472,733,000	320,406,000	471,495,000	474,016,000	414,297,000	276,429,000	451,904,000	398,189,000	1,738,650,000	1,540,819,000	1,373,113,000					11,700,000																142,771,000	48,368,000	104,514,000	237,685,000	124,406,000	61,809,000	70,887,000	194,487,000
Operating income (loss) from continuing operations	110,630,000	(4,052,000)	142,433,000	102,190,000	111,609,000	13,964,000	215,847,000	120,494,000	351,201,000	461,914,000	393,699,000					3,100,000																34,800,000	(22,544,000)	2,219,000	79,293,000	22,951,000	(12,743,000)	4,448,000	78,560,000	59,081,000	62,972,000	59,546,000
Recognized amounts of identifiable assets acquired and liabilities assumed
Proved properties															65,581,000							36,068,000				151,544,000
Unproved leasehold properties															911,000							23,686,000				7,883,000
Accounts receivable															1,358,000							680,000				3,070,000
Accounts payable															(25,000)							(244,000)				(388,000)
Asset retirement obligation															(210,000)							(173,000)				(142,000)
Total identifiable net assets															$ 67,615,000							$ 60,017,000				$ 161,967,000