Income Taxes (Deferred Tax Assets and Liabilities) (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Line Items]
Full valuation allowance	$ 3,100,000
Deferred tax assets:
Total deferred tax assets, Current	33,455,000	50,557,000
Total deferred tax assets, Noncurrent	22,760,000	15,850,000
Valuation allowance, Current	(268,000)	(270,000)
Valuation allowance, Noncurrent	(2,793,000)	(2,752,000)
Total deferred tax assets, net, Current	33,187,000	50,287,000
Total deferred tax assets, net, Noncurrent	19,967,000	13,098,000
Deferred tax liabilities:
Total deferred tax liabilities, Current	24,667,000	1,469,000
Total deferred tax liabilities, Noncurrent	925,568,000	819,225,000
Net deferred tax assets (liabilities), Current	8,520,000	48,818,000
Net deferred tax assets (liabilities), Noncurrent	(905,601,000)	(806,127,000)
Alabama Gas Corporation
Deferred tax assets:
Total deferred tax assets, net, Current	20,200,000	23,334,000
Total deferred tax assets, net, Noncurrent	2,000	4,000
Deferred tax liabilities:
Total deferred tax liabilities, Current	1,401,000	1,348,000
Total deferred tax liabilities, Noncurrent	189,383,000	181,496,000
Net deferred tax assets (liabilities), Current	18,799,000	21,986,000
Net deferred tax assets (liabilities), Noncurrent	(189,381,000)	(181,492,000)
Oil and Gas Operations
Income Tax Disclosure [Line Items]
State net operating loss carryforwards, subject to expiration	12,200,000
Deferred tax assets, Current
Deferred tax assets:
Unbilled and deferred revenue	10,137,000	9,582,000
Allowance for doubtful accounts	2,408,000	4,848,000
Insurance accruals	2,021,000	2,562,000
Compensation accruals	13,116,000	11,181,000
Inventories	1,664,000	1,438,000
Other comprehensive income	0	2,799,000
Gas supply adjustment related accruals	969,000	1,196,000
Derivative instruments	0	13,167,000
State net operating losses and other carryforwards	0	987,000
Other	3,140,000	2,797,000
Deferred tax assets, Current | Alabama Gas Corporation
Deferred tax assets:
Unbilled and deferred revenue	10,137,000	9,582,000
Allowance for doubtful accounts	2,155,000	4,575,000
Insurance accruals	1,856,000	2,358,000
Compensation accruals	2,645,000	2,274,000
Inventories	1,664,000	1,438,000
Gas supply adjustment related accruals	969,000	1,196,000
State net operating losses and other carryforwards	0	987,000
Other	774,000	924,000
Deferred tax assets, Noncurrent
Income Tax Disclosure [Line Items]
Deferred Tax Assets, State Operating Loss Carryforwards and Other Tax Carryforwards, Portion About To Expire	500,000
Deferred tax assets:
Unbilled and deferred revenue	0	0
Allowance for doubtful accounts	0	0
Insurance accruals	0	0
Compensation accruals	0	0
Inventories	0	0
Other comprehensive income	19,158,000	12,801,000
Gas supply adjustment related accruals	0	0
Derivative instruments	0	0
State net operating losses and other carryforwards	3,577,000	3,022,000
Other	25,000	27,000
Deferred tax assets, Noncurrent | Alabama Gas Corporation
Deferred tax assets:
Unbilled and deferred revenue	0	0
Allowance for doubtful accounts	0	0
Insurance accruals	0	0
Compensation accruals	0	0
Inventories	0	0
Gas supply adjustment related accruals	0	0
State net operating losses and other carryforwards	0	0
Other	2,000	4,000
Deferred tax liabilities, Current
Deferred tax liabilities:
Depreciation and basis differences	0	0
Pension and other costs	0	0
Derivative instruments	4,272,000	0
Other comprehensive income	18,133,000	0
Other	2,262,000	1,469,000
Deferred tax liabilities, Current | Alabama Gas Corporation
Deferred tax liabilities:
Depreciation and basis differences	0	0
Pension and other costs	0	0
Other	1,401,000	1,348,000
Deferred tax liabilities, Noncurrent
Deferred tax liabilities:
Depreciation and basis differences	898,625,000	791,073,000
Pension and other costs	20,143,000	25,685,000
Derivative instruments	3,162,000	0
Other comprehensive income	0	0
Other	3,638,000	2,467,000
Deferred tax liabilities, Noncurrent | Alabama Gas Corporation
Deferred tax liabilities:
Depreciation and basis differences	167,329,000	156,121,000
Pension and other costs	22,054,000	25,375,000
Other	$ 0	$ 0