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, 2012	Dec. 31, 2011	Dec. 31, 2012 Refundable negative salvage	Dec. 31, 2012 Alabama Gas Corporation	Dec. 31, 2011 Alabama Gas Corporation	Dec. 31, 2012 Alabama Gas Corporation Refundable negative salvage	Dec. 31, 2011 Alabama Gas Corporation Refundable negative salvage	Dec. 31, 2010 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, 2011 Alabama Gas Corporation APSC Approved Reduction in Depreciation Rates, June 1, 2010	Dec. 31, 2010 Alabama Gas Corporation APSC Approved Reduction in Depreciation Rates, 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	Dec. 31, 2010 Alabama Gas Corporation Depreciation Rate, Prior to APSC Approved Reduction, June 1, 2010
Property, Plant and Equipment [Line Items]
Composite depreciation rate											3.10%			3.20%	3.10%	3.60%
Refunded negative salvage costs								$ 2,700,000	$ 14,200,000	$ 22,200,000	$ 25,600,000
Regulatory Liability, Current	45,116,000	58,279,000	18,300,000	45,116,000	58,279,000	18,265,000	20,269,000
Regulatory Liability, Noncurrent	80,404,000	87,234,000	53,500,000	80,404,000	87,234,000	53,467,000	65,646,000
Refundable negative salvage costs, term						7 years
Total amount refundable to customers, term						9 years
Decrease in depreciation expense												6,800,000	9,200,000
Gas imbalances					$ 500,000