Discontinued Operations and Held for Sale Properties - Balance Sheet and Income Statement (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Sep. 02, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 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 From Discontinued Operations		0	$ 436,620	0	$ 469,490
Income (loss) from discontinued operations		0	(3,485)	0	30,435
Natural gas distribution revenues			39,874		397,648
Oil and natural gas revenues			6		5,217
Total revenues			39,880		402,865
Pretax income (loss) from discontinued operations			(5,938)		48,581
Income tax expense (benefit)			(2,453)		18,146
Loss on disposal of discontinued operations			726,410		724,743
Income tax benefit			286,305		285,688
Loss on Disposal of Discontinued Operations		$ 0	$ 440,105	$ 0	$ 439,055
Diluted Earnings Per Average Common Share
Income from discontinued operations (in dollars per share)			$ (0.05)		$ 0.42
Loss on disposal of discontinued operations (in dollars per share)			5.99		5.99
Total Income (Loss) From Discontinued Operations (in dollars per share)		$ 0.00	5.94	$ 0.00	6.41
Basic Earnings Per Average Common Share
Income from discontinued operations (in dollars per share)			(0.05)		0.42
Loss on disposal of discontinued operations (in dollars per share)			6.03		6.03
Total Income (Loss) From Discontinued Operations (in dollars per share)		$ 0.00	$ 5.98	$ 0.00	$ 6.45
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)			$ (1,767)		$ 40,646
Depreciation, depletion and amortization			(103)		(408)
General and administrative			1,710		3,337
Interest expense			(4,329)		(17,306)
Other income			0		(347)
Income tax expense			1,029		5,567
Income From Discontinued Operations			(3,460)		31,489
Loss on disposal of discontinued operations	$ 726,500
Parent Company
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income From Discontinued Operations			$ (25)		$ (1,054)