Income Taxes (Components of Income Taxes) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Taxes estimated to be payable currently:
Federal	$ 23,342	$ 16,295	$ 11,595
State	2,516	3,125	5,065
Total current	25,858	19,420	16,660
Taxes deferred:
Federal	85,950	119,053	125,622
State	(2,300)	5,346	3,419
Total deferred	83,650	124,399	129,041
Total income tax expense	109,508	143,819	145,701
Taxes deferred:
Total income tax expense	105,282	144,534	126,322
Alabama Gas Corporation
Taxes estimated to be payable currently:
Federal	17,495	18,227	(1,280)
State	2,192	739	(108)
Total current	19,687	18,966	(1,388)
Taxes deferred:
Federal	13,252	9,066	24,938
State	1,748	2,212	3,120
Total deferred	15,000	11,278	28,058
Total income tax expense	$ 34,687	$ 30,244	$ 26,670