Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of December 31, 2012			$ 118,023
Liabilities incurred			1,914
Liabilities settled			(448)
Accretion expense	2,043	1,861	4,040	3,674
Reclassification associated with held for sale properties			(4,500)
Balance as of June 30, 2013	119,029		119,029
Regulatory assets, noncurrent	107,189		107,189		110,566
Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of December 31, 2012					24,900
Balance as of June 30, 2013	25,500		25,500		24,900
Regulatory assets, noncurrent	107,189		107,189		110,566
Asset removal costs | Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory assets, noncurrent	$ 2,328		$ 2,328		$ 3,322