CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock	Premimum on Capital Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Deferred Compensation Plan	Treasury Stock	Alabama Gas Corporation	Alabama Gas Corporation Common Stock	Alabama Gas Corporation Premimum on Capital Stock [Member]	Alabama Gas Corporation Capital Surplus [Member]	Alabama Gas Corporation Retained Earnings [Member]
BALANCE at Dec. 31, 2009	$ 1,988,243	$ 746	$ 461,661	$ 2,802	$ 1,626,753	$ 17,615	$ 3,121	$ (124,455)	$ 317,803	$ 20	$ 31,682	$ 2,802	$ 283,299
BALANCE, shares at Dec. 31, 2009		74,593,431								1,972,052
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	290,807				290,807				46,883				46,883
Comprehensive Income	(92,012)					(92,012)
Purchase of treasury shares, net	(2,893)							(2,893)
Shares issued for employee benefit plans, shares		192,945
Shares issued for employee benefit plans	6,451	2	6,449
Deferred compensation obligation	0						167	(167)
Stock based compensation	(83)		(83)
Tax benefit from employee stock plans	907		907
Cash dividends, per share - ($0.50 in 2009, $0.52 in 2010, $0.54 in 2011)	(37,377)				(37,377)				(37,367)				(37,367)
BALANCE at Dec. 31, 2010	2,154,043	748	468,934	2,802	1,880,183	(74,397)	3,288	(127,515)	327,319	20	31,682	2,802	292,815
BALANCE, shares at Dec. 31, 2010		74,786,376								1,972,052
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	94,268								44,175
BALANCE at Mar. 31, 2011
BALANCE at Dec. 31, 2010	2,154,043	748	468,934	2,802	1,880,183	(74,397)	3,288	(127,515)	327,319	20	31,682	2,802	292,815
BALANCE, shares at Dec. 31, 2010		74,786,376								1,972,052
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	259,624				259,624				46,602				46,602
Comprehensive Income	44,145					44,145
Purchase of treasury shares, net	(713)							(713)
Shares issued for employee benefit plans, shares		221,036
Shares issued for employee benefit plans	7,237	2	7,235
Deferred compensation obligation	0						223	(223)
Stock based compensation	5,763		5,763
Tax benefit from employee stock plans	986		986
Cash dividends, per share - ($0.50 in 2009, $0.52 in 2010, $0.54 in 2011)	(38,922)				(38,922)				(29,183)				(29,183)
BALANCE at Dec. 31, 2011	2,432,163	750	482,918	2,802	2,100,885	(30,252)	3,511	(128,451)	344,738	20	31,682	2,802	310,234
BALANCE, shares at Dec. 31, 2011	75,007,412	75,007,412							1,972,052	1,972,052
BALANCE at Sep. 30, 2011
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	14,432								11,258
BALANCE at Dec. 31, 2011	2,432,163			2,802					344,738	20	31,682	2,802
BALANCE, shares at Dec. 31, 2011	75,007,412								1,972,052	1,972,052
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	57,406								46,918
BALANCE at Mar. 31, 2012
BALANCE at Dec. 31, 2011	2,432,163	750	482,918	2,802	2,100,885	(30,252)	3,511	(128,451)	344,738	20	31,682	2,802	310,234
BALANCE, shares at Dec. 31, 2011	75,007,412	75,007,412							1,972,052	1,972,052
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	253,562				253,562				49,402				49,402
Comprehensive Income	21,941					21,941
Purchase of treasury shares, net	(277)							(277)
Shares issued for employee benefit plans, shares		60,348
Shares issued for employee benefit plans	2,061	1	2,060
Deferred compensation obligation	0						(737)	737
Stock based compensation	7,082		6,580					502
Tax benefit from employee stock plans	550		550
Cash dividends, per share - ($0.50 in 2009, $0.52 in 2010, $0.54 in 2011)	(40,392)				(40,392)				(33,637)				(33,637)
BALANCE at Dec. 31, 2012	2,676,690	751	492,108	2,802	2,314,055	(8,311)	2,774	(127,489)	360,503	20	31,682	2,802	325,999
BALANCE, shares at Dec. 31, 2012	75,067,760	75,067,760							1,972,052	1,972,052
BALANCE at Sep. 30, 2012
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	62,823								12,197
BALANCE at Dec. 31, 2012	$ 2,676,690			$ 2,802					$ 360,503	$ 20	$ 31,682	$ 2,802
BALANCE, shares at Dec. 31, 2012	75,067,760								1,972,052	1,972,052