Income Taxes (Unrecognized Tax Benefits) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	$ 24,590,000	$ 17,797,000	$ 16,805,000
Additions based on tax posititons related to the current year	3,644,000	1,436,000	2,530,000
Additions for tax positions of prior years	2,324,000	11,703,000	841,000
Reductions for tax positions of prior years (lapse of statue of limitations)			(2,379,000)
Reductions for tax positions of prior years	(39,000)	(3,624,000)
Lapse of statute of limitations	(1,482,000)	(1,313,000)
Settlements	(18,444,000)	(1,409,000)
Unrecognized Tax Benefits, Ending Balance	10,593,000	24,590,000	17,797,000
Gross addition (reduction)	(5,900,000)	3,600,000
Unrecognized income tax benefit liability, additions (release)	2,900,000	(2,400,000)
Full amount of tax benefit, chance of being realized, percentage	50.00%
Liability (benefit) adjustment due to settlement with IRS	(1,500,000)
Unrecognized tax benefits that would favorably impact effective tax rate if recognized	4,100,000
Income tax interest (net of tax benefit) and penalties expense (income)	(1,400,000)	800,000	100,000
Accrued interest (net of tax benefit) and penalties payments	200,000	1,600,000
Alabama Gas Corporation
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	18,941,000	7,621,000	6,890,000
Additions based on tax posititons related to the current year	13,000	9,000	821,000
Additions for tax positions of prior years	1,000	11,645,000	197,000
Reductions for tax positions of prior years (lapse of statue of limitations)	(409,000)	(90,000)	(287,000)
Settlements	(18,444,000)	(244,000)
Unrecognized Tax Benefits, Ending Balance	102,000	18,941,000	7,621,000
Unrecognized tax benefits that would favorably impact effective tax rate if recognized	16,000
Income tax interest (net of tax benefit) and penalties expense (income)	(1,400,000)	1,000,000	100,000
Accrued interest (net of tax benefit) and penalties payments	$ 5,000	$ 1,400,000