SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Other Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 740	$ 741
Accumulated Amortization	(328)	(280)
Other Intangible Assets, Net	412	461
Intangible assets, gross	815	816
Intangible assets, net	487	536
Federal Communications Commission licenses
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	75	75
Accumulated Amortization	0	0
Other Intangible Assets, Net	75	75
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	212	212
Accumulated Amortization	(135)	(116)
Other Intangible Assets, Net	77	96
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(31)	(25)
Other Intangible Assets, Net	33	39
Storage and transportation contracts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(64)	(62)
Other Intangible Assets, Net	0	2
PPA fair value adjustments
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	390	390
Accumulated Amortization	(89)	(69)
Other Intangible Assets, Net	301	321
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	10	11
Accumulated Amortization	(9)	(8)
Other Intangible Assets, Net	1	3
Southern Power
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	(89)	(69)
Southern Power | PPA fair value adjustments
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	390	390
Accumulated Amortization	(89)	(69)
Other Intangible Assets, Net	301	321
Southern Company Gas
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	246	246
Accumulated Amortization	(195)	(176)
Other Intangible Assets, Net	51	70
Intangible assets, net	51	70
Southern Company Gas | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	156	156
Accumulated Amortization	(119)	(104)
Other Intangible Assets, Net	37	52
Southern Company Gas | Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	26	26
Accumulated Amortization	(12)	(10)
Other Intangible Assets, Net	14	16
Southern Company Gas | Storage and transportation contracts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(64)	(62)
Other Intangible Assets, Net	$ 0	$ 2