Summarized Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Selected Quarterly Financial Information [Abstract]
Operating revenues	$ 288,148	[1]	$ 378,568	$ 330,399	$ 486,364	$ 374,091	$ 295,804	$ 333,725	$ 574,914	$ 1,483,479	$ 1,578,534	$ 1,440,420
Operating income	31,641	[1]	150,412	106,335	159,881	135,566	67,320	98,293	192,198	448,269	493,377	435,393
Net income	14,432	[1]	87,599	63,325	94,268	80,250	38,304	55,543	116,710	259,624	290,807	256,325
Diluted Earnings Per Average Common Share (dollars per share)	$ 0.20	[1]	$ 1.21	$ 0.87	$ 1.30	$ 1.11	$ 0.53	$ 0.77	$ 1.62	$ 3.59	$ 4.04	$ 3.57
Basic Earnings Per Average Common Share (dollars per share)	$ 0.20	[1]	$ 1.22	$ 0.88	$ 1.31	$ 1.12	$ 0.53	$ 0.77	$ 1.63	$ 3.60	$ 4.05	$ 3.58
After-tax non-cash mark-to-market loss on derivatives	56,600									37,600
After-tax non-cash mark-to-market loss on derivatives, per diluted shares	$ 0.78
Alabama Gas Corporation
Selected Quarterly Financial Information [Abstract]
Operating revenues	119,456		59,616	86,309	269,572	121,640	61,693	99,139	337,300
Operating income	20,675		(10,681)	1,163	75,059	19,005	(9,015)	3,138	75,255	59,546	58,508	56,631
Net income	$ 11,258		$ (9,093)	$ 262	$ 44,175	$ 10,097	$ (7,120)	$ (340)	$ 44,246	$ 46,602	$ 46,883	$ 45,415

[1] The three months ended December 31, 2011 includes an after-tax non-cash mark-to-market loss on derivatives of $56.6 million, or $0.78 per diluted share.