Noncontrolling Interest - Redeemable Noncontrolling Interest (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Noncontrolling Interest [Roll Forward]
Purchase of membership interests from noncontrolling interests		$ (129)
SOUTHERN POWER CO
Noncontrolling Interest [Roll Forward]
Beginning balance	$ 164	43	$ 39
Net income attributable to redeemable noncontrolling interests	2	4	2
Distributions to redeemable noncontrolling interests	(2)	(1)	0
Capital contributions from redeemable noncontrolling interests	2	118	2
Redeemable Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	59	0
Purchase of membership interests from noncontrolling interests		(129)	0
Reclassification to non-redeemable noncontrolling interests	(114)	0	0
Change in fair value of redeemable noncontrolling interests	7	0	0
Ending balance	$ 0	$ 164	$ 43