Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance of ARO, beginning			$ 107,340
Liabilities incurred			2,946
Liabilities settled			(676)
Accretion expense	1,907	1,728	5,581	5,066
Balance of ARO, end	115,191		115,191
Regulatory Assets, Noncurrent	94,869		94,869		95,633
Regulatory Liability, Current	27,372		27,372		58,279
Regulatory Liability, Noncurrent	77,692		77,692		87,234
Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance of ARO, beginning					20,800
Balance of ARO, end	21,500		21,500		20,800
Regulatory Assets, Noncurrent	94,869		94,869		95,633
Regulatory Liability, Current	27,372		27,372		58,279
Regulatory Liability, Noncurrent	77,692		77,692		87,234
Asset removal costs | Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory Assets, Noncurrent					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	55,400		55,400		65,600
Refundable negative salvage | Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory Liability, Current	18,353		18,353		20,269
Regulatory Liability, Noncurrent	$ 55,389		$ 55,389		$ 65,646