Income Taxes (Deferred Tax Assets and Liabilities) (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Line Items]
Full valuation allowance	$ 3,000,000
Book basis in excess of tax basis for subsidiary	37,000,000
Deferred tax liability, book basis in excess of tax basis in subsidiary	14,000,000
Deferred tax assets:
Total deferred tax assets, Current	48,815,000	33,455,000
Total deferred tax assets, Noncurrent	19,928,000	22,760,000
Valuation allowance, Current	(299,000)	(268,000)
Valuation allowance, Noncurrent	(2,674,000)	(2,793,000)
Total deferred tax assets, net, Current	48,516,000	33,187,000
Total deferred tax assets, net, Noncurrent	17,254,000	19,967,000
Deferred tax liabilities:
Total deferred tax liabilities, Current	7,217,000	24,667,000
Total deferred tax liabilities, Noncurrent	1,030,499,000	925,568,000
Net deferred tax assets (liabilities), Current	41,299,000	8,520,000
Net deferred tax assets (liabilities), Noncurrent	(1,013,245,000)	(905,601,000)
Alabama Gas Corporation
Deferred tax assets:
Total deferred tax assets, net, Current	21,553,000	20,200,000
Total deferred tax assets, net, Noncurrent	1,000	2,000
Deferred tax liabilities:
Total deferred tax liabilities, Current	1,504,000	1,401,000
Total deferred tax liabilities, Noncurrent	205,632,000	189,383,000
Net deferred tax assets (liabilities), Current	20,049,000	18,799,000
Net deferred tax assets (liabilities), Noncurrent	(205,631,000)	(189,381,000)
Oil and Gas Operations
Income Tax Disclosure [Line Items]
Deferred Tax Assets, State Operating Loss Carryforwards and Other Tax Carryforwards, Portion About To Expire	35,000,000
Deferred tax assets, Current
Deferred tax assets:
Unbilled and deferred revenue	12,547,000	10,137,000
Allowance for doubtful accounts	2,066,000	2,408,000
Insurance and other accruals	4,851,000	3,821,000
Compensation accruals	15,405,000	13,116,000
Inventories	1,260,000	1,664,000
Other comprehensive income	0	0
Gas supply adjustment related accruals	698,000	969,000
Derivative instruments	10,769,000	0
State net operating losses and other carryforwards	0	0
Other	1,219,000	1,340,000
Deferred tax assets, Current | Alabama Gas Corporation
Deferred tax assets:
Unbilled and deferred revenue	12,547,000	10,137,000
Allowance for doubtful accounts	1,815,000	2,155,000
Insurance and other accruals	1,769,000	1,856,000
Compensation accruals	2,480,000	2,645,000
Inventories	1,260,000	1,664,000
Gas supply adjustment related accruals	698,000	969,000
Other	984,000	774,000
Deferred tax assets, Noncurrent
Income Tax Disclosure [Line Items]
State net operating loss carryforwards, subject to expiration	1,600,000
Deferred tax assets:
Unbilled and deferred revenue	0	0
Allowance for doubtful accounts	0	0
Insurance and other accruals	0	0
Compensation accruals	0	0
Inventories	0	0
Other comprehensive income	15,350,000	19,158,000
Gas supply adjustment related accruals	0	0
Derivative instruments	0	0
State net operating losses and other carryforwards	4,577,000	3,577,000
Other	1,000	25,000
Deferred tax assets, Noncurrent | Alabama Gas Corporation
Deferred tax assets:
Unbilled and deferred revenue	0	0
Allowance for doubtful accounts	0	0
Insurance and other accruals	0	0
Compensation accruals	0	0
Inventories	0	0
Gas supply adjustment related accruals	0	0
Other	1,000	2,000
Deferred tax liabilities, Current
Deferred tax liabilities:
Depreciation and basis differences	0	0
Pension and other costs	0	0
Derivative instruments	0	4,272,000
Other comprehensive income	5,540,000	18,133,000
Other	1,677,000	2,262,000
Deferred tax liabilities, Current | Alabama Gas Corporation
Deferred tax liabilities:
Depreciation and basis differences	0	0
Pension and other costs	0	0
Other	1,504,000	1,401,000
Deferred tax liabilities, Noncurrent
Deferred tax liabilities:
Depreciation and basis differences	1,008,026,000	898,625,000
Pension and other costs	15,379,000	20,143,000
Derivative instruments	2,048,000	3,162,000
Other comprehensive income	0	0
Other	5,046,000	3,638,000
Deferred tax liabilities, Noncurrent | Alabama Gas Corporation
Deferred tax liabilities:
Depreciation and basis differences	186,601,000	167,329,000
Pension and other costs	19,031,000	22,054,000
Other	$ 0	$ 0