Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 11,213	$ 45,515
Regulatory assets, noncurrent	89,004	110,566
Regulatory Liability, Current	40,168	45,116
Regulatory Liability, Noncurrent	81,414	80,404
Alabama Gas Corporation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	11,213	45,515
Regulatory assets, noncurrent	89,004	110,566
Regulatory Liability, Current	40,168	45,116
Regulatory Liability, Noncurrent	81,414	80,404
Alabama Gas Corporation | RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	6,573	1,740
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Unbilled service margin
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	6,351	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	13,951	1,237
Alabama Gas Corporation | Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	16,321	18,265
Regulatory Liability, Noncurrent	40,946	53,467
Alabama Gas Corporation | Gain on sale of property
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	10,890	0
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Accretion and depreciation for asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	25,772	24,930
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	33	33
Regulatory Liability, Noncurrent	745	770
Alabama Gas Corporation | Pension and postretirement assets
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	263	170
Regulatory assets, noncurrent	69,249	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	17,579	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	2,176	3,322
Alabama Gas Corporation | Gas supply adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	10,925	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