Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 57,143	$ 28,286
Regulatory Assets, Noncurrent	95,633	105,365
Regulatory Liability, Current	58,279	75,703
Regulatory Liability, Noncurrent	87,234	110,447
Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	20,300
Regulatory Liability, Noncurrent	65,600
Alabama Gas Corporation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	57,143	28,286
Regulatory Assets, Noncurrent	95,633	105,365
Regulatory Liability, Current	58,279	75,703
Regulatory Liability, Noncurrent	87,234	110,447
Alabama Gas Corporation | RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	2,931	4,147
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Unbilled service margin
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	22,419	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	12,626	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	0	6,913
Alabama Gas Corporation | Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	20,269	22,336
Regulatory Liability, Noncurrent	65,646	90,504
Alabama Gas Corporation | Asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	20,785	11,439
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	34	33
Regulatory Liability, Noncurrent	803	837
Alabama Gas Corporation | Pension and postretirement assets
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	170	132
Regulatory Assets, Noncurrent	77,587	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	13,981	8,681
Alabama Gas Corporation | Risk management activities
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	56,804	27,906
Regulatory Assets, Noncurrent	3,070	32,461
Alabama Gas Corporation | Asset removal costs, net
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Noncurrent	994	0
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 | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	169	248
Regulatory Assets, Noncurrent	$ 1	$ 145