Accumulated Other Comprehensive Income (Loss) (Reclassifications of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Gains (losses) on cash flow hedges:
Income (Loss) From Continuing Operations Before Income Taxes										$ (1,480,736)	$ 140,371	$ 216,204
Income tax expense										535,005	(40,728)	(74,323)
Income (Loss) From Continuing Operations		$ (590,806)	$ (227,904)	$ (111,601)	$ (15,420)	$ 66,519	$ 20,631	$ (3,154)	$ 15,647	(945,731)	99,643	141,881
Pension and postretirement plans:
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax										(19,828)	(2,514)	12,324
Nonqualified Supplemental Retirement Plans
Pension and postretirement plans:
Settlement charges					$ 2,500	$ 1,800					400	600
Settlement charges expensed												200
Nonqualified Supplemental Retirement Plans | Alabama Gas Corporation
Pension and postretirement plans:
Settlement charges												400
Transition obligation
Pension and postretirement plans:
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax										0	(22)	(319)
Prior service cost
Pension and postretirement plans:
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax										0	(248)	(257)
Actuarial losses
Pension and postretirement plans:
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax										(30,504)	(21,932)	(12,357)
Actuarial losses on settlement charges
Pension and postretirement plans:
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1]									0	0	(421)
Pension and Postretirement Plans
Gains (losses) on cash flow hedges:
Income tax expense										10,676	7,771	4,674
Pension and postretirement plans:
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax										(30,504)	(22,202)	(13,354)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax										(19,828)	(14,431)	(8,680)
Reclassification out of Accumulated Other Comprehensive Income | Cash Flow Hedges
Gains (losses) on cash flow hedges:
Income (Loss) From Continuing Operations Before Income Taxes										0	19,331	33,961
Income tax expense										0	(7,414)	(12,957)
Income (Loss) From Continuing Operations										0	11,917	21,004
Reclassification out of Accumulated Other Comprehensive Income | Commodity contracts | Cash Flow Hedges
Gains (losses) on cash flow hedges:
Commodity contracts										0	21,611	35,684
Reclassification out of Accumulated Other Comprehensive Income | Interest rate swap | Cash Flow Hedges
Gains (losses) on cash flow hedges:
Interest rate swap										$ 0	$ (2,280)	$ (1,723)

[1] During the year ended December 31, 2013, Energen incurred settlement charges of $0.6 million for the payment of lump sums from the nonqualified supplemental retirement plans, of which $0.2 million was recognized in actuarial losses above and $0.4 million was recognized as a regulatory asset at Alagasco and reported in actuarial losses on settlement charges above.