Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 2,756	$ 45,515
Regulatory Assets, Noncurrent	84,890	110,566
Regulatory Liability, Current	49,006	45,116
Regulatory Liability, Noncurrent	94,125	80,404
Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	15,800
Regulatory Liability, Noncurrent	39,700
Gas supply adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	2,406	42,726
Regulatory Assets, Noncurrent	0	0
Alabama Gas Corporation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	2,756	45,515
Regulatory Assets, Noncurrent	84,890	110,566
Regulatory Liability, Current	49,006	45,116
Regulatory Liability, Noncurrent	94,125	80,404
Alabama Gas Corporation | RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	4,690	1,740
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Unbilled service margin
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	28,504	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	26,197	1,237
Alabama Gas Corporation | Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	15,779	18,265
Regulatory Liability, Noncurrent	39,663	53,467
Alabama Gas Corporation | Asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	27,528	24,930
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	33	33
Regulatory Liability, Noncurrent	737	770
Alabama Gas Corporation | Pension assets
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	325	170
Regulatory Assets, Noncurrent	58,243	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	18,046	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	4,601	3,322
Alabama Gas Corporation | Enhanced stability reserve
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Noncurrent	4,000	0
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	25	26
Regulatory Assets, Noncurrent	$ 0	$ 0