INCOME TAXES - Current and Deferred Income Tax Provisions (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Federal —
Current		$ 156	$ 167	$ (62)
Deferred		1,237	231	(6)
Total federal taxes		1,393	398	(68)
State —
Current		275	188	37
Deferred		130	(137)	173
Total state taxes		405	51	210
Income taxes		1,798	449	142
Alabama Power
Federal —
Current		61	91	136
Deferred		125	123	336
Total federal taxes		186	214	472
State —
Current		12	26	23
Deferred		72	51	73
Total state taxes		84	77	96
Income taxes		270	291	568
Georgia Power
Federal —
Current		264	393	256
Deferred		180	(249)	504
Total federal taxes		444	144	760
State —
Current		6	81	116
Deferred		22	(11)	(46)
Total state taxes		28	70	70
Income taxes		472	214	830
Mississippi Power
Federal —
Current		(6)	(567)	194
Deferred		26	575	(753)
Total federal taxes		20	8	(559)
State —
Current		(1)	(10)	0
Deferred		11	(100)	27
Total state taxes		10	(110)	27
Income taxes		30	(102)	(532)
Southern Power
Federal —
Current		(717)	85	(566)
Deferred		647	(154)	(312)
Total federal taxes		(70)	(69)	(878)
State —
Current		1	(9)	(110)
Deferred		13	(86)	49
Total state taxes		14	(95)	(61)
Income taxes	$ 75	(56)	(164)	(939)
Southern Company Gas
Federal —
Current		(120)	334	103
Deferred		195	33	170
Total federal taxes		75	367	273
State —
Current		37	131	27
Deferred		18	(34)	67
Total state taxes		55	97	94
Income taxes		$ 130	$ 464	$ 367