Discontinued Operations and Held for Sale Properties - Balance Sheet and Income Statement (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
	Sep. 02, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Assets of Disposal Group, Including Discontinued Operation [Abstract]
Total liabilities held for sale		$ 0		$ 0		$ (24,230)
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income (Loss) From Discontinued Operations		0	$ (4,799)	0	$ 32,870
Income (loss) from discontinued operations		0	(4,799)	0	33,920
Natural gas distribution revenues			93,873		357,774
Oil and natural gas revenues			390		5,211
Total revenues			94,263		362,985
Pretax income (loss) from discontinued operations			(7,797)		54,519
Income tax expense (benefit)			(2,998)		20,599
Loss on disposal of discontinued operations			0		(1,667)
Income tax benefit			0		(617)
Loss on Disposal of Discontinued Operations		$ 0	$ 0	$ 0	$ (1,050)
Diluted Earnings Per Average Common Share
Income from discontinued operations (in dollars per share)			$ (0.07)		$ 0.46
Loss on disposal of discontinued operations (in dollars per share)			0.00		(0.01)
Total Income (Loss) From Discontinued Operations (in dollars per share)		$ 0.00	(0.07)	$ 0.00	0.45
Basic Earnings Per Average Common Share
Income from discontinued operations (in dollars per share)			(0.07)		0.46
Loss on disposal of discontinued operations (in dollars per share)			0.00		(0.01)
Total Income (Loss) From Discontinued Operations (in dollars per share)		$ 0.00	$ (0.07)	$ 0.00	$ 0.45
Net change in credit facility				$ (352,000)	$ 180,000
San Juan Basin
Assets of Disposal Group, Including Discontinued Operation [Abstract]
Oil and natural gas properties						1,166,124
Less accumulated depreciation, depletion and amortization						(770,327)
Total assets held for sale						395,797
Other long-term liabilities						(24,230)
Total liabilities held for sale						(24,230)
Total net assets held for sale						$ 371,567
Alabama Gas Corporation
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Alagasco net income (loss)			$ (615)		42,413
Depreciation, depletion and amortization			(153)		(305)
General and administrative			687		1,627
Interest expense			(7,190)		(12,977)
Other income			(228)		(347)
Income tax expense			2,603		4,538
Income (Loss) From Discontinued Operations			(4,896)		34,949
Loss on disposal of discontinued operations	$ 726,500
Parent Company
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income (Loss) From Discontinued Operations			$ 97		$ (1,029)