Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of December 31, 2012	$ 118,023
Liabilities incurred	901
Liabilities settled	(322)
Accretion expense	1,997	1,813
Balance as of March 31, 2013	120,599
Regulatory assets, noncurrent	109,002		110,566
Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of December 31, 2012			24,900
Balance as of March 31, 2013	25,200		24,900
Regulatory assets, noncurrent	109,002		110,566
Asset removal costs | Alabama Gas Corporation
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory assets, noncurrent	$ 2,802		$ 3,322