Income Taxes (Components of Income Taxes) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Taxes estimated to be payable currently:
Federal	$ 11,595	$ 31,920	$ 56,821
State	5,065	1,230	2,534
Total current	16,660	33,150	59,355
Taxes deferred:
Federal	125,622	121,804	75,644
State	3,419	12,036	8,972
Total deferred	129,041	133,840	84,616
Total income tax expense	145,701	166,990	143,971
Alabama Gas Corporation
Taxes estimated to be payable currently:
Federal	(1,280)	859	11,035
State	(108)	155	61
Total current	(1,388)	1,014	11,096
Taxes deferred:
Federal	24,938	25,582	13,631
State	3,120	3,279	2,626
Total deferred	28,058	28,861	16,257
Total income tax expense	$ 26,670	$ 29,875	$ 27,353