Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance of ARO, beginning	$ 97,415	$ 88,298	$ 66,151
Liabilities incurred	4,627	4,033	8,226
Liabilities settled	(1,539)	(1,094)	(672)
Revision in estimated cash flows			9,658
Accretion expense	6,837	6,178	4,935
Balance of ARO, end	107,340	97,415	88,298
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets, Noncurrent	95,633	105,365
Regulatory Liability, Current	58,279	75,703
Regulatory Liability, Noncurrent	87,234	110,447
Jun 30, 2009, Permian Basin
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Liabilities incurred			6,600
Refundable negative salvage
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Liability, Current	20,300
Regulatory Liability, Noncurrent	65,600
Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance of ARO, end	20,800	11,400
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets, Noncurrent	95,633	105,365
Regulatory Liability, Current	58,279	75,703
Regulatory Liability, Noncurrent	87,234	110,447
Alabama Gas Corporation | Asset removal costs
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	0	6,913
Alabama Gas Corporation | Refundable negative salvage
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Liability, Current	20,269	22,336
Regulatory Liability, Noncurrent	65,646	90,504
Alabama Gas Corporation | Asset removal costs
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets, Noncurrent	$ 994	$ 0