Summary of Significant Accounting Policies (Utility Plant and Depreciation) (Details) (USD $)	12 Months Ended									1 Months Ended		12 Months Ended			0 Months Ended	12 Months Ended
	Dec. 31, 2013	Jun. 30, 2013 bbl	Mar. 31, 2013 bbl	Dec. 31, 2012	Dec. 31, 2013 Refundable negative salvage	Dec. 31, 2013 Alabama Gas Corporation	Dec. 31, 2012 Alabama Gas Corporation	Dec. 31, 2013 Alabama Gas Corporation Refundable negative salvage	Dec. 31, 2012 Alabama Gas Corporation Refundable negative salvage	Dec. 31, 2010 Alabama Gas Corporation Refundable negative salvage Reduction in Tariff Rates	Jul. 31, 2010 Alabama Gas Corporation Refundable negative salvage Reduction in Tariff Rates	Dec. 31, 2013 Alabama Gas Corporation Refundable negative salvage Reduction in Tariff Rates	Dec. 31, 2012 Alabama Gas Corporation Refundable negative salvage Reduction in Tariff Rates	Dec. 31, 2011 Alabama Gas Corporation Refundable negative salvage Reduction in Tariff Rates	Jun. 01, 2010 Alabama Gas Corporation APSC Approved Reduction in Depreciation Rates, June 1, 2010	Dec. 31, 2013 Alabama Gas Corporation Depreciation Rate, Prior to APSC Approved Reduction, June 1, 2010	Dec. 31, 2012 Alabama Gas Corporation Depreciation Rate, Prior to APSC Approved Reduction, June 1, 2010	Dec. 31, 2011 Alabama Gas Corporation Depreciation Rate, Prior to APSC Approved Reduction, June 1, 2010
Property, Plant and Equipment [Line Items]
Derivatives transferred to not designated as hedging instruments		2,353,000	5,078,000
Composite depreciation rate															3.10%	3.10%	3.20%	3.10%
Refunded negative salvage costs										$ 22,200,000	$ 25,600,000	$ 16,300,000	$ 14,200,000	$ 2,700,000
Regulatory Liability, Current	49,006,000			45,116,000	15,800,000	49,006,000	45,116,000	15,779,000	18,265,000
Regulatory Liability, Noncurrent	$ 94,125,000			$ 80,404,000	$ 39,700,000	$ 94,125,000	$ 80,404,000	$ 39,663,000	$ 53,467,000
Refundable negative salvage costs, term	12 months