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Income Taxes - Current and Deferred Income Tax Provisions (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Federal —
Current				$ 167	$ (62)	$ 1,184
Deferred				231	(6)	(342)
Total federal taxes				398	(68)	842
State —
Current				188	37	(108)
Deferred				(137)	173	217
Total state taxes				51	210	109
Income taxes				449	142	951
Alabama Power
Federal —
Current				91	136	103
Deferred				123	336	339
Total federal taxes				214	472	442
State —
Current				26	23	20
Deferred				51	73	69
Total state taxes				77	96	89
Income taxes				291	568	531
Georgia Power
Federal —
Current				393	256	391
Deferred				(249)	504	319
Total federal taxes				144	760	710
State —
Current				81	116	6
Deferred				(11)	(46)	64
Total state taxes				70	70	70
Income taxes				214	830	780
Mississippi Power
Federal —
Current				(567)	194	(31)
Deferred				575	(753)	(60)
Total federal taxes				8	(559)	(91)
State —
Current				(10)	0	(6)
Deferred				(100)	27	(7)
Total state taxes				(110)	27	(13)
Income taxes				(102)	(532)	(104)
Southern Power
Federal —
Current				85	(566)	928
Deferred				(154)	(312)	(1,098)
Total federal taxes				(69)	(878)	(170)
State —
Current				(9)	(110)	(60)
Deferred				(86)	49	35
Total state taxes				(95)	(61)	(25)
Income taxes	$ 75			(164)	(939)	$ (195)
Southern Company Gas
Federal —
Current		$ 0		334	103
Deferred		65		33	170
Total federal taxes		65		367	273
State —
Current		(16)		131	27
Deferred		27		(34)	67
Total state taxes		11		97	94
Income taxes		$ 76		$ 464	$ 367
Predecessor \| Southern Company Gas
Federal —
Current			$ 67
Deferred			8
Total federal taxes			75
State —
Current			12
Deferred			0
Total state taxes			12
Income taxes			$ 87