Discontinued Operations and Held for Sale Properties (Income Statement) (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Sep. 02, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Natural gas distribution revenues			$ 397,648	$ 533,338
Oil and natural gas revenues			5,199	60,191
Total revenues			402,847	593,529
Pretax income from discontinued operations			47,220	92,253
Income tax expense		$ 0	17,928	33,174
Income From Discontinued Operations		0	29,292	59,079
Gain on disposal of discontinued operations, net			724,594	5,605
Income tax expense		0	285,497	2,011
Gain on Disposal of Discontinued Operations, net		0	439,097	3,594
Income From Discontinued Operations		$ 0	$ 468,389	$ 62,673
Diluted earnings per average common share
Income from discontinued operations			$ 0.40	$ 0.81
Gain on disposal of discontinued operations, net			5.99	0.05
Total Income From Discontinued Operations		$ 0.00	6.39	0.86
Basic earnings per average common share
Income from discontinued operations			0.40	0.82
Gain on disposal of discontinued operations, net			6.02	0.05
Total Income From Discontinued Operations		$ 0.00	$ 6.42	$ 0.87
Alabama Gas Corporation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Alagasco net income			$ 40,646	$ 57,399
Depreciation, depletion and amortization			(408)	(598)
General and administrative			3,337	5,894
Interest expense			(17,306)	(13,815)
Other income			(347)	(1,342)
Income tax expense			5,567	3,728
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain on disposal of discontinued operations, net	$ 726,500
Income From Discontinued Operations			31,489	51,266
Parent Company
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income From Discontinued Operations			$ (2,197)	$ 7,813