Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of December 31, 2013	$ 108,533
Liabilities incurred	766
Liabilities settled	(413)
Accretion expense	1,843	1,687
Balance as of March 31, 2014	110,729
Regulatory assets, noncurrent	82,570		84,890
Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of December 31, 2013			27,500
Balance as of March 31, 2014	27,800		27,500
Regulatory assets, noncurrent	82,570		84,890
Alabama Gas Corporation | Asset removal costs
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory assets, noncurrent	$ 4,001		$ 4,601