Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 56,188	$ 57,143
Regulatory Assets, Noncurrent	94,869	95,633
Regulatory Liability, Current	27,372	58,279
Regulatory Liability, Noncurrent	77,692	87,234
Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	18,400	20,300
Regulatory Liability, Noncurrent	55,400	65,600
Alabama Gas Corporation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	56,188	57,143
Regulatory Assets, Noncurrent	94,869	95,633
Regulatory Liability, Current	27,372	58,279
Regulatory Liability, Noncurrent	77,692	87,234
Alabama Gas Corporation | RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	3,187	2,931
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Unbilled service margin
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	5,799	22,419
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Gas Supply Adjustment [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	12,626
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	18,353	20,269
Regulatory Liability, Noncurrent	55,389	65,646
Alabama Gas Corporation | Asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	21,525	20,785
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	33	34
Regulatory Liability, Noncurrent	778	803
Alabama Gas Corporation | Pension and postretirement assets
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	170	170
Regulatory Assets, Noncurrent	79,984	77,587
Alabama Gas Corporation | Accretion and depreciation for asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Noncurrent	14,885	13,981
Alabama Gas Corporation | Risk management activities
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	14,990	56,804
Regulatory Assets, Noncurrent	0	3,070
Alabama Gas Corporation | Asset removal costs, net
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Noncurrent	0	994
Alabama Gas Corporation | Gas Supply Adjustment [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	40,998	0
Regulatory Assets, Noncurrent	0	0
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	30	169
Regulatory Assets, Noncurrent	$ 0	$ 1