Income Taxes (Deferred Tax Assets and Liabilities) (Details) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Income Tax Disclosure [Line Items]
Full valuation allowance	$ 3,022,000
Deferred tax assets:
Total deferred tax assets, Current	50,557,000	37,677,000
Total deferred tax assets, Noncurrent	15,850,000	49,611,000
Valuation allowance, Current	(270,000)	(311,000)
Valuation allowance, Noncurrent	(2,752,000)	(2,555,000)
Total deferred tax assets, net, Current	50,287,000	37,366,000
Total deferred tax assets, net, Noncurrent	13,098,000	47,056,000
Deferred tax liabilities:
Total deferred tax liabilities, Current	1,469,000	4,634,000
Total deferred tax liabilities, Noncurrent	819,225,000	662,140,000
Net deferred tax assets (liabilities), Current	48,818,000	32,732,000
Net deferred tax assets (liabilities), Noncurrent	(806,127,000)	(615,084,000)
Alabama Gas Corporation
Income Tax Disclosure [Line Items]
State net operating loss carryforwards, subject to expiration	22,800,000
Deferred tax assets:
Total deferred tax assets, net, Current	23,334,000	28,546,000
Total deferred tax assets, net, Noncurrent	4,000	55,000
Deferred tax liabilities:
Total deferred tax liabilities, Current	1,348,000	1,244,000
Total deferred tax liabilities, Noncurrent	181,496,000	141,835,000
Net deferred tax assets (liabilities), Current	21,986,000	27,302,000
Net deferred tax assets (liabilities), Noncurrent	(181,492,000)	(141,780,000)
Deferred tax assets, Current
Deferred tax assets:
Unbilled and deferred revenue	9,582,000	14,495,000
Allowance for doubtful accounts	4,848,000	5,626,000
Insurance accruals	2,562,000	2,350,000
Compensation accruals	11,181,000	9,822,000
Inventories	1,438,000	896,000
Other comprehensive income	2,799,000	0
Gas supply adjustment related accruals	1,196,000	1,407,000
Derivative instruments	13,167,000	0
State net operating losses and other carryforwards	987,000	811,000
Other	2,797,000	2,270,000
Deferred tax assets, Current | Alabama Gas Corporation
Deferred tax assets:
Unbilled and deferred revenue	9,582,000	14,495,000
Allowance for doubtful accounts	4,575,000	5,369,000
Insurance accruals	2,358,000	2,143,000
Compensation accruals	2,274,000	2,672,000
Inventories	1,438,000	896,000
Gas supply adjustment related accruals	1,196,000	1,407,000
State net operating losses and other carryforwards	987,000	811,000
Other	924,000	753,000
Deferred tax assets, Noncurrent
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	12,801,000	46,244,000
Gas supply adjustment related accruals	0	0
Derivative instruments	0	0
State net operating losses and other carryforwards	3,022,000	2,866,000
Other	27,000	501,000
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	4,000	55,000
Deferred tax liabilities, Current
Deferred tax liabilities:
Depreciation and basis differences	0	0
Pension and other costs	0	0
Derivative instruments	0	333,000
Other comprehensive income	0	2,934,000
Other	1,469,000	1,367,000
Deferred tax liabilities, Current | Alabama Gas Corporation
Deferred tax liabilities:
Depreciation and basis differences	0	0
Pension and other costs	0	0
Other	1,348,000	1,244,000
Deferred tax liabilities, Noncurrent
Deferred tax liabilities:
Depreciation and basis differences	791,073,000	632,032,000
Pension and other costs	25,685,000	29,437,000
Derivative instruments	0	0
Other comprehensive income	0	0
Other	2,467,000	671,000
Deferred tax liabilities, Noncurrent | Alabama Gas Corporation
Deferred tax liabilities:
Depreciation and basis differences	156,121,000	114,193,000
Pension and other costs	25,375,000	27,642,000
Other	$ 0	$ 0