Income Taxes (Components of Income Taxes) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Taxes estimated to be payable currently:
Federal	$ 16,295	$ 11,595	$ 31,920
State	3,125	5,065	1,230
Total current	19,420	16,660	33,150
Taxes deferred:
Federal	119,053	125,622	121,804
State	5,346	3,419	12,036
Total deferred	124,399	129,041	133,840
Total income tax expense	143,819	145,701	166,990
Alabama Gas Corporation
Taxes estimated to be payable currently:
Federal	18,227	(1,280)	859
State	739	(108)	155
Total current	18,966	(1,388)	1,014
Taxes deferred:
Federal	9,066	24,938	25,582
State	2,212	3,120	3,279
Total deferred	11,278	28,058	28,861
Total income tax expense	$ 30,244	$ 26,670	$ 29,875