Regulatory Assets and Liabilities (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 44,525	$ 28,286
Regulatory Assets, Noncurrent	109,244	105,365
Regulatory Liability, Current	51,849	75,703
Regulatory Liability, Noncurrent	87,579	110,447
Alabama Gas Corporation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	44,525	28,286
Regulatory Assets, Noncurrent	109,244	105,365
Regulatory Liability, Current	51,849	75,703
Regulatory Liability, Noncurrent	87,579	110,447
Alabama Gas Corporation | Enhanced stability reserve
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Noncurrent	0	3,794
Alabama Gas Corporation | Pension and postretirement assets
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	170	132
Regulatory Assets, Noncurrent	87,181	60,284
Alabama Gas Corporation | Accretion and depreciation for asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Noncurrent	9,007	8,681
Alabama Gas Corporation | Risk management activities
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	44,137	27,906
Regulatory Assets, Noncurrent	13,050	32,461
Alabama Gas Corporation | RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	25	25
Regulatory Assets, Noncurrent	0	0
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	193	223
Regulatory Assets, Noncurrent	6	145
Alabama Gas Corporation | RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	5,068	4,147
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Unbilled service margin
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	5,143	34,197
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Postretirement liabilities
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	0	754
Alabama Gas Corporation | Gas supply adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	21,060	14,990
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Asset removal costs, net
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	7,834	6,913
Alabama Gas Corporation | Asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	11,697	11,439
Alabama Gas Corporation | Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	20,545	22,336
Regulatory Liability, Noncurrent	67,237	90,504
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	33	33
Regulatory Liability, Noncurrent	811	837
Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	20,545
Regulatory Liability, Noncurrent	$ 67,237