Summarized Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Information [Abstract]
Operating revenues	$ 433,046	$ 295,324	$ 470,355	$ 418,444	$ 288,148	$ 378,568	$ 330,399	$ 486,364	$ 1,617,169	$ 1,483,479	$ 1,578,534
Operating income (loss)	115,166	19,458	220,598	104,170	31,641	150,412	106,335	159,881	459,392	448,269	493,377
Net Income	62,823	2,046	131,287	57,406	14,432	87,599	63,325	94,268	253,562	259,624	290,807
Diluted earnings per average common share	$ 0.87	$ 0.03	$ 1.82	$ 0.79	$ 0.20	$ 1.21	$ 0.87	$ 1.30	$ 3.51	$ 3.59	$ 4.04
Basic earnings per average common share	$ 0.87	$ 0.03	$ 1.82	$ 0.80	$ 0.20	$ 1.22	$ 0.88	$ 1.31	$ 3.52	$ 3.60	$ 4.05
Alabama Gas Corporation
Selected Quarterly Financial Information [Abstract]
Operating revenues	124,406	61,809	70,887	194,487	119,456	59,616	86,309	269,572
Operating income (loss)	22,951	(12,743)	4,448	78,560	20,675	(10,681)	1,163	75,059	62,972	59,546	58,508
Net Income	$ 12,197	$ (10,039)	$ 326	$ 46,918	$ 11,258	$ (9,093)	$ 262	$ 44,175	$ 49,402	$ 46,602	$ 46,883