Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance of ARO, beginning			$ 107,340
Liabilities incurred			2,122
Liabilities settled			(343)
Accretion expense	1,861	1,689	3,674	3,338
Balance of ARO, end	112,793		112,793
Regulatory Assets, Noncurrent	89,785		89,785		95,633
Regulatory Liability, Current	28,497		28,497		58,279
Regulatory Liability, Noncurrent	77,144		77,144		87,234
Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance of ARO, beginning					20,800
Balance of ARO, end	21,300		21,300		20,800
Regulatory Assets, Noncurrent	89,785		89,785		95,633
Regulatory Liability, Current	28,497		28,497		58,279
Regulatory Liability, Noncurrent	77,144		77,144		87,234
Asset removal costs | Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory Assets, Noncurrent	100		100		1,000
Refundable negative salvage
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory Liability, Current	18,400		18,400		20,300
Regulatory Liability, Noncurrent	55,100		55,100		65,600
Refundable negative salvage | Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory Liability, Current	18,360		18,360		20,269
Regulatory Liability, Noncurrent	$ 55,079		$ 55,079		$ 65,646