Income Taxes (Effective Income Tax Rate Reconciliation) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
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	$ 104,450	$ 139,914	$ 122,719
State income taxes, net of federal income tax benefit	3,799	4,755	8,341
Impact of state law changes	(1,966)	0	(2,059)
Qualified Section 199 production activities deduction	0	(61)	(495)
401(k) stock dividend deduction	(449)	(514)	(532)
Other, net	(552)	440	(1,652)
Total income tax expense	105,282	144,534	126,322
Effective income tax rate (%)	35.28%	36.16%	36.03%
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	32,230	27,876	25,645
State income taxes, net of federal income tax benefit	2,588	2,238	2,059
Reversal of tax reserves from audit settlements, net	0	0	(1,365)
Other, net	(131)	130	331
Total income tax expense	$ 34,687	$ 30,244	$ 26,670
Effective income tax rate (%)	37.67%	37.97%	36.40%