Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 1,283	$ 2,756
Regulatory assets, noncurrent	82,570	84,890
Regulatory Liability, Current	80,698	49,006
Regulatory Liability, Noncurrent	83,240	94,125
Alabama Gas Corporation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	1,283	2,756
Regulatory assets, noncurrent	82,570	84,890
Regulatory Liability, Current	80,698	49,006
Regulatory Liability, Noncurrent	83,240	94,125
Alabama Gas Corporation | RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	16,193	4,690
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Unbilled service margin
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	28,251	28,504
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Postretirement liabilities
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	25,826	26,197
Alabama Gas Corporation | Gas supply adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	23,782	0
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	12,439	15,779
Regulatory Liability, Noncurrent	28,846	39,663
Alabama Gas Corporation | Accretion and depreciation of asset retirement obligations
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	27,840	27,528
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	33	33
Regulatory Liability, Noncurrent	728	737
Alabama Gas Corporation | Pension assets
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	1,258	325
Regulatory assets, noncurrent	56,133	58,243
Alabama Gas Corporation | Accretion and depreciation of asset retirement obligations
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory assets, noncurrent	18,436	18,046
Alabama Gas Corporation | Rate recovery of asset removal costs, net
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory assets, noncurrent	4,001	4,601
Alabama Gas Corporation | Enhanced stability reserve
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory assets, noncurrent	4,000	4,000
Alabama Gas Corporation | Gas supply adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	2,406
Regulatory assets, noncurrent	0	0
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	25	25
Regulatory assets, noncurrent	$ 0	$ 0