HELD FOR SALE PROPERTIES AND DISCONTINUED OPERATIONS - Income Statement (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Sep. 02, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Natural gas distribution revenues				$ 397,648
Oil and natural gas revenues				5,199
Total revenues				402,847
Pretax income from discontinued operations				47,220
Income tax expense		$ 0	$ 0	17,928
Income From Discontinued Operations		0	0	29,292
Gain on disposal of discontinued operations, net				724,594
Income tax expense		0	0	285,497
Gain on Disposal of Discontinued Operations, net		0	0	439,097
Income From Discontinued Operations		$ 0	$ 0	$ 468,389
Diluted earnings per average common share
Income from discontinued operations (in dollars per share)				$ 0.40
Gain on disposal of discontinued operations, net (in dollars per share)				5.99
Total Income From Discontinued Operations (in dollars per share)		$ 0.00	$ 0.00	6.39
Basic earnings per average common share
Income from discontinued operations (in dollars per share)				0.40
Gain on disposal of discontinued operations, net (in dollars per share)				6.02
Total Income From Discontinued Operations (in dollars per share)		$ 0.00	$ 0.00	$ 6.42
Alabama Gas Corporation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Alagasco net income				$ 40,646
Depreciation, depletion and amortization				(408)
General and administrative				3,337
Interest expense				(17,306)
Other income				(347)
Income tax expense				5,567
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Gain on disposal of discontinued operations, net	$ 726,500
Income From Discontinued Operations				31,489
Parent Company
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income From Discontinued Operations				$ (2,197)