Regulatory Assets and Liabilities (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 32,563	$ 28,286
Regulatory Assets, Noncurrent	89,465	105,365
Regulatory Liability, Current	65,024	75,703
Regulatory Liability, Noncurrent	92,951	110,447
Alabama Gas Corporation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	32,563	28,286
Regulatory Assets, Noncurrent	89,465	105,365
Regulatory Liability, Current	65,024	75,703
Regulatory Liability, Noncurrent	92,951	110,447
Alabama Gas Corporation | Enhanced stability reserve
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Noncurrent	4,395	3,794
Alabama Gas Corporation | Pension and postretirement assets
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	170	132
Regulatory Assets, Noncurrent	58,469	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	8,898	8,681
Alabama Gas Corporation | Risk management activities
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	32,159	27,906
Regulatory Assets, Noncurrent	17,657	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	209	223
Regulatory Assets, Noncurrent	46	145
Alabama Gas Corporation | RSE adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	7,696	4,147
Regulatory Liability, Noncurrent	0	0
Alabama Gas Corporation | Unbilled service margin
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	6,957	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	959	754
Alabama Gas Corporation | Gas supply adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	29,695	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	7,665	6,913
Alabama Gas Corporation | Asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	11,611	11,439
Alabama Gas Corporation | Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	20,643	22,336
Regulatory Liability, Noncurrent	71,896	90,504
Alabama Gas Corporation | Other
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	33	33
Regulatory Liability, Noncurrent	820	837
Refundable negative salvage
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	20,643
Regulatory Liability, Noncurrent	$ 71,896