Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance of ARO, beginning	$ 107,340
Liabilities incurred	1,260
Liabilities settled	(204)
Accretion expense	1,813	1,649
Balance of ARO, end	110,209
Regulatory Liability, Current	30,049		58,279
Regulatory Liability, Noncurrent	74,159		87,234
Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance of ARO, beginning			20,800
Balance of ARO, end	21,000		20,800
Regulatory Liability, Current	30,049		58,279
Regulatory Liability, Noncurrent	74,159		87,234
Asset removal costs | Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory Liability, Noncurrent	700		1,000
Refundable negative salvage
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory Liability, Current	21,900		20,300
Regulatory Liability, Noncurrent	52,300		65,600
Refundable negative salvage | Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory Liability, Current	21,914		20,269
Regulatory Liability, Noncurrent	$ 52,333		$ 65,646