Income Taxes (Effective Income Tax Rate Reconciliation) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Income Tax Disclosure [Line Items]
Statutory federal income tax rate	35.00%	35.00%	35.00%
Income tax expense at statutory federal income tax rate	$ 141,864	$ 160,229	$ 140,104
State income taxes, net of federal income tax benefit	5,544	8,398	7,384
Qualified Section 199 production activities deduction	(593)	(1,745)	(2,715)
401(k) stock dividend deduction	(532)	(565)	(567)
Other, net	(582)	673	(235)
Total income tax expense	145,701	166,990	143,971
Effective income tax rate (%)	35.95%	36.48%	35.97%
Alabama Gas Corporation
Income Tax Disclosure [Line Items]
Statutory federal income tax rate	35.00%	35.00%	35.00%
Income tax expense at statutory federal income tax rate	25,645	26,865	25,469
State income taxes, net of federal income tax benefit	2,059	2,157	2,045
Reversal of tax reserves from audit settlements, net	(1,365)	0	0
Other, net	331	853	(161)
Total income tax expense	$ 26,670	$ 29,875	$ 27,353
Effective income tax rate (%)	36.40%	38.92%	37.59%