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, 2011	Dec. 31, 2010	Dec. 31, 2011 Refundable negative salvage	Dec. 31, 2011 Alabama Gas Corporation	Dec. 31, 2010 Alabama Gas Corporation	Dec. 31, 2011 Alabama Gas Corporation Refundable negative salvage	Dec. 31, 2010 Alabama Gas Corporation Refundable negative salvage	Jul. 31, 2010 Alabama Gas Corporation Refundable negative salvage One-time Bill Credit	Dec. 31, 2010 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, 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	Dec. 31, 2009 Alabama Gas Corporation Depreciation Rate, Prior to APSC Approved Reduction, June 1, 2010
Property, Plant and Equipment [Line Items]
Composite depreciation rate											3.10%			3.10%	3.60%	4.40%
Refunded negative salvage costs								$ 25,600,000	$ 2,700,000	$ 22,200,000
Regulatory Liability, Current	58,279,000	75,703,000	20,300,000	58,279,000	75,703,000	20,269,000	22,336,000
Regulatory Liability, Noncurrent	87,234,000	110,447,000	65,600,000	87,234,000	110,447,000	65,646,000	90,504,000
Refundable negative salvage costs, term						8 years
Total amount refundable to customers, term						9 years
Decrease in depreciation expense												6,800,000	9,200,000
Gas imbalances				$ 500,000