Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 60,485	$ 57,143
Regulatory Assets, Noncurrent	89,785	95,633
Regulatory Liability, Current	28,497	58,279
Regulatory Liability, Noncurrent	77,144	87,234
Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	18,400	20,300
Regulatory Liability, Noncurrent	55,100	65,600
Alabama Gas Corporation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	60,485	57,143
Regulatory Assets, Noncurrent	89,785	95,633
Regulatory Liability, Current	28,497	58,279
Regulatory Liability, Noncurrent	77,144	87,234
Alabama Gas Corporation | RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	4,586	2,931
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Unbilled service margin
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	5,518	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	18,360	20,269
Regulatory Liability, Noncurrent	55,079	65,646
Alabama Gas Corporation | Asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	21,278	20,785
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	33	34
Regulatory Liability, Noncurrent	787	803
Alabama Gas Corporation | Pension and postretirement assets
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	170	170
Regulatory Assets, Noncurrent	74,742	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	14,584	13,981
Alabama Gas Corporation | Risk management activities
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	30,573	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	126	994
Alabama Gas Corporation | Gas Supply Adjustment [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	29,671	0
Regulatory Assets, Noncurrent	0	0
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	71	169
Regulatory Assets, Noncurrent	$ 333	$ 1