Income Taxes (Unrecognized Tax Benefits) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	$ 10,593,000	$ 24,590,000	$ 17,797,000
Additions based on tax positions related to the current year	3,731,000	3,644,000	1,436,000
Additions for tax positions of prior years	269,000	2,324,000	11,703,000
Reductions for tax positions of prior years (lapse of statute of limitations)			(3,624,000)
Reductions for tax positions of prior years	(446,000)	(39,000)
Lapse of statute of limitations	(1,592,000)	(1,482,000)	(1,313,000)
Settlements		(18,444,000)	(1,409,000)
Unrecognized Tax Benefits, Ending Balance	12,555,000	10,593,000	24,590,000
Gross addition (reduction)		5,900,000	(3,600,000)
Unrecognized income tax benefit liability, additions (release)	(5,100,000)	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
Income tax interest (net of tax benefit) and penalties expense (income)	0	1,400,000	800,000
Accrued interest (net of tax benefit) and penalties payments	200,000	200,000
Alabama Gas Corporation
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	102,000	18,941,000	7,621,000
Additions based on tax positions related to the current year	62,000	13,000	9,000
Additions for tax positions of prior years	201,000	1,000	11,645,000
Reductions for tax positions of prior years (lapse of statute of limitations)	(58,000)	(409,000)	(90,000)
Settlements		(18,444,000)	(244,000)
Unrecognized Tax Benefits, Ending Balance	307,000	102,000	18,941,000
Income tax interest (net of tax benefit) and penalties expense (income)	1,000	1,400,000	1,000,000
Accrued interest (net of tax benefit) and penalties payments	$ 4,000	$ 5,000