Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 24,135	$ 45,515
Regulatory assets, noncurrent	109,002	110,566
Regulatory Liability, Current	41,886	45,116
Regulatory Liability, Noncurrent	68,812	80,404
Alabama Gas Corporation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	24,135	45,515
Regulatory assets, noncurrent	109,002	110,566
Regulatory Liability, Current	41,886	45,116
Regulatory Liability, Noncurrent	68,812	80,404
Alabama Gas Corporation | RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	2,262	1,740
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Unbilled service margin
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	22,108	25,078
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Postretirement liabilities
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	1,487	1,237
Alabama Gas Corporation | Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	17,483	18,265
Regulatory Liability, Noncurrent	41,352	53,467
Alabama Gas Corporation | Accretion and depreciation for asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	25,211	24,930
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	33	33
Regulatory Liability, Noncurrent	762	770
Alabama Gas Corporation | Pension and postretirement assets
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	253	170
Regulatory assets, noncurrent	89,316	90,708
Alabama Gas Corporation | Accretion and depreciation for asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory assets, noncurrent	16,884	16,536
Alabama Gas Corporation | Risk management activities
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	2,593
Regulatory assets, noncurrent	0	0
Alabama Gas Corporation | Rate recovery of asset removal costs, net
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory assets, noncurrent	2,802	3,322
Alabama Gas Corporation | Gas supply adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	23,857	42,726
Regulatory assets, noncurrent	0	0
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	25	26
Regulatory assets, noncurrent	$ 0	$ 0