Summarized Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected Quarterly Financial Information [Abstract]
Adjusted operating revenues	$ 472,733		$ 320,406		$ 471,495		$ 474,016		$ 414,297	$ 276,429	$ 451,904	$ 398,189	$ 1,738,650	$ 1,540,819	$ 1,373,113
Oil and gas revenues	0	[1]	0	[1]	(18,562)	[1]	(18,663)	[1]	(18,749)	(18,895)	(18,451)	(20,255)	60,191	76,350	110,366
Operating income (loss)	110,630		(4,052)		142,433		102,190		111,609	13,964	215,847	120,494	351,201	461,914	393,699
Income (loss) from continuing operations	63,325		(5,486)		80,614		54,694		60,552	(1,505)	128,305	67,868
Discontinued operations	0	[1]	0	[1]	(3,871)	[1]	(3,146)	[1]	(3,557)	(5,494)	(4,751)	16,324
Net Income	84,093		(19,298)		83,067		56,692		62,823	2,046	131,287	57,406	204,554	253,562	259,624
Diluted earnings per average common share
Continuing operations (in dollars per share)	$ 0.87		$ (0.08)		$ 1.11		$ 0.76		$ 0.84	$ (0.02)	$ 1.77	$ 0.94	$ 2.67	$ 3.53	$ 3.10
Net income (loss) (in dollars per share)	$ 1.15		$ (0.27)		$ 1.15		$ 0.78		$ 0.87	$ 0.03	$ 1.82	$ 0.79	$ 2.82	$ 3.51	$ 3.59
Basic earnings per average common share
Continuing operations (in dollars per share)	$ 0.87		$ (0.08)		$ 1.12		$ 0.76		$ 0.84	$ (0.02)	$ 1.78	$ 0.94	$ 2.67	$ 3.54	$ 3.11
Net income (loss) (in dollars per share)	$ 1.16		$ (0.27)		$ 1.15		$ 0.79		$ 0.87	$ 0.03	$ 1.82	$ 0.80	$ 2.83	$ 3.52	$ 3.60
Alabama Gas Corporation
Selected Quarterly Financial Information [Abstract]
Adjusted operating revenues	142,771		48,368		104,514		237,685		124,406	61,809	70,887	194,487
Operating income (loss)	34,800		(22,544)		2,219		79,293		22,951	(12,743)	4,448	78,560	59,081	62,972	59,546
Net Income	19,842		(8,961)		(704)		47,222		12,197	(10,039)	326	46,918	57,399	49,402	46,602
Originally reported
Selected Quarterly Financial Information [Abstract]
Adjusted operating revenues	472,733		320,406		490,057		492,679		433,046	295,324	470,355	418,444
Operating income (loss)	$ 110,630		$ (4,052)		$ 146,304		$ 105,336		$ 115,166	$ 19,458	$ 220,598	$ 104,170

[1] As discussed in Note 13, Discontinued Operations, during the fourth quarter of 2013, the Company completed the sale of its Black Warrior Basin coalbed methane properties in Alabama. The property was classified as held-for-sale and reflected in discontinued operations during the third quarter of 2013. Also, during the third quarter of 2013, the Company classified its North Louisiana/East Texas natural gas and oil properties as held-for-sale and reflected the associated operating results in discontinued operations.