Discontinued Operations (Balance Sheet) (Details) (USD $)	Dec. 31, 2013		Dec. 31, 2014
Assets of Disposal Group, Including Discontinued Operation [Abstract]
Cash	$ 3,032,000
Accounts receivable	107,849,000	[1]
Inventories	41,268,000
Oil and natural gas properties	348,379,000
Less accumulated depreciation, depletion and amortization	(301,609,000)
Utility plant	1,491,433,000
Less accumulated depreciation	(605,924,000)
Other property, net	206,000
Other current assets	29,458,000	[1]
Other long-term assets	128,780,000
Total assets held-for-sale	1,242,872,000
Liabilities of Disposal Group, Including Discontinued Operation [Abstract]
Accounts payable	(50,396,000)
Royalty payable	(1,419,000)
Accrued taxes	(28,027,000)
Notes payable to banks	(50,000,000)
Other current liabilities	(105,413,000)	[1]
Other long-term liabilities	(346,392,000)
Long-term debt	(249,923,000)
Total liabilities held-for-sale	(831,570,000)
Total held-for-sale properties	411,302,000
Adjustment for affiliated companies receivables	4,700,000
Assets reclassified to continuing operations	1,600,000
Liabilities reclassified to continuing operations	500,000
San Juan Basin
Assets of Disposal Group, Including Discontinued Operation [Abstract]
Oil and natural gas properties			1,166,124,000
Less accumulated depreciation, depletion and amortization			(770,327,000)
Total assets held-for-sale			395,797,000
Liabilities of Disposal Group, Including Discontinued Operation [Abstract]
Other long-term liabilities			(24,230,000)
Total liabilities held-for-sale			(24,230,000)
Total held-for-sale properties			371,567,000
Alabama Gas Corporation
Assets of Disposal Group, Including Discontinued Operation [Abstract]
Cash	3,032,000
Accounts receivable	103,748,000	[1]
Inventories	41,200,000
Oil and natural gas properties	0
Less accumulated depreciation, depletion and amortization	0
Utility plant	1,491,433,000
Less accumulated depreciation	(605,924,000)
Other property, net	41,000
Other current assets	29,458,000	[1]
Other long-term assets	128,780,000
Total assets held-for-sale	1,191,768,000
Liabilities of Disposal Group, Including Discontinued Operation [Abstract]
Accounts payable	(48,653,000)
Royalty payable	0
Accrued taxes	(28,027,000)
Notes payable to banks	(50,000,000)
Other current liabilities	(105,013,000)	[1]
Other long-term liabilities	(331,409,000)
Long-term debt	(249,923,000)
Total liabilities held-for-sale	(813,025,000)
Total held-for-sale properties	378,743,000
Black Warrior Basin
Assets of Disposal Group, Including Discontinued Operation [Abstract]
Cash	0
Accounts receivable	2,829,000	[1]
Inventories	0
Oil and natural gas properties	0
Less accumulated depreciation, depletion and amortization
Utility plant	0
Less accumulated depreciation	0
Other property, net	0
Other current assets	0	[1]
Other long-term assets	0
Total assets held-for-sale	2,829,000
Liabilities of Disposal Group, Including Discontinued Operation [Abstract]
Accounts payable	(1,732,000)
Royalty payable	(550,000)
Accrued taxes	0
Notes payable to banks	0
Other current liabilities	(379,000)	[1]
Other long-term liabilities	0
Long-term debt	0
Total liabilities held-for-sale	(2,661,000)
Total held-for-sale properties	168,000
North Louisiana/East Texas
Assets of Disposal Group, Including Discontinued Operation [Abstract]
Cash	0
Accounts receivable	1,272,000	[1]
Inventories	68,000
Oil and natural gas properties	348,379,000
Less accumulated depreciation, depletion and amortization	(301,609,000)
Utility plant	0
Less accumulated depreciation	0
Other property, net	165,000
Other current assets	0	[1]
Other long-term assets	0
Total assets held-for-sale	48,275,000
Liabilities of Disposal Group, Including Discontinued Operation [Abstract]
Accounts payable	(11,000)
Royalty payable	(869,000)
Accrued taxes	0
Notes payable to banks	0
Other current liabilities	(21,000)	[1]
Other long-term liabilities	(14,983,000)
Long-term debt	0
Total liabilities held-for-sale	(15,884,000)
Total held-for-sale properties	$ 32,391,000

[1] At December 31, 2013, Alagasco’s accounts receivable included a consolidating adjustment of $4.7 million to adjust for affiliated companies receivables. Certain other current assets and other current liabilities at Alagasco of $1.6 million and $0.5 million, respectively, were reclassified to continuing operations at Energen.