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, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance			$ 164
Reclassification of noncontrolling interest to contingently redeemable noncontrolling interest			(114)
Net income attributable to noncontrolling interests			46	$ 36	$ 14
Distributions to noncontrolling interests			(119)	(72)	(18)
Ending balance	$ 164		0	164
SOUTHERN Co GAS
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance	174		0
Net income attributable to noncontrolling interests	0		0
Distributions to noncontrolling interests	(15)		$ 0
Reclassification of contingently redeemable noncontrolling interest to mandatorily redeemable noncontrolling interest	(174)
Ending balance	0	$ 174		0
SOUTHERN Co GAS | Predecessor
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
Distributions to noncontrolling interests		(19)			(18)
Ending balance		$ 41			$ 0