Income Taxes (Effective Income Tax Rate Reconciliation) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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	$ 139,083	$ 141,864	$ 160,229
State income taxes, net of federal income tax benefit	4,904	5,544	8,398
Qualified Section 199 production activities deduction	(94)	(593)	(1,745)
401(k) stock dividend deduction	(514)	(532)	(565)
Other, net	440	(582)	673
Total income tax expense	143,819	145,701	166,990
Effective income tax rate (%)	36.19%	35.95%	36.48%
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	27,876	25,645	26,865
State income taxes, net of federal income tax benefit	2,238	2,059	2,157
Reversal of tax reserves from audit settlements, net	0	(1,365)	0
Other, net	130	331	853
Total income tax expense	$ 30,244	$ 26,670	$ 29,875
Effective income tax rate (%)	37.97%	36.40%	38.92%