Joint Ownership Agreements - Redeemable Noncontrolling Interest Roll Forward (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance		$ 43	$ 43
Net income attributable to noncontrolling interests			36	$ 14	$ 0
Distributions to noncontrolling interests			(72)	(18)	(1)
Ending balance	$ 164		164	43
Southern Company Gas [Member] | Predecessor [Member]
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance	41	0	0
Reclassification of noncontrolling interest to contingently redeemable noncontrolling interest		46
Net income attributable to noncontrolling interests		14		20	18
Distributions to noncontrolling interests		(19)		(18)	$ (17)
Ending balance		41		$ 0
Southern Company Gas [Member] | Successor [Member]
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance	174
Net income attributable to noncontrolling interests	0
Distributions to noncontrolling interests	(15)
Reclassification of contingently redeemable noncontrolling interest to mandatorily redeemable noncontrolling interest	(174)
Ending balance	$ 0	$ 174	$ 0