INTRODUCTION - Other Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Other intangible assets subject to amortization:
Gross Carrying Amount	$ 740	$ 741
Accumulated Amortization	(316)	(280)
Other Intangible Assets, Net	424	461
Other intangible assets not subject to amortization:
Federal Communications Commission licenses	75	75
Total other intangible assets
Gross Carrying Amount	815	816
Accumulated Amortization	(316)	(280)
Other Intangible Assets, Net	499	536
Customer relationships
Other intangible assets subject to amortization:
Gross Carrying Amount	212	212
Accumulated Amortization	(130)	(116)
Other Intangible Assets, Net	82	96
Total other intangible assets
Accumulated Amortization	(130)	(116)
Trade names
Other intangible assets subject to amortization:
Gross Carrying Amount	64	64
Accumulated Amortization	(30)	(25)
Other Intangible Assets, Net	34	39
Total other intangible assets
Accumulated Amortization	(30)	(25)
Storage and transportation contracts
Other intangible assets subject to amortization:
Gross Carrying Amount	64	64
Accumulated Amortization	(64)	(62)
Other Intangible Assets, Net	0	2
Total other intangible assets
Accumulated Amortization	(64)	(62)
PPA fair value adjustments
Other intangible assets subject to amortization:
Gross Carrying Amount	390	390
Accumulated Amortization	(84)	(69)
Other Intangible Assets, Net	306	321
Total other intangible assets
Accumulated Amortization	(84)	(69)
Other
Other intangible assets subject to amortization:
Gross Carrying Amount	10	11
Accumulated Amortization	(8)	(8)
Other Intangible Assets, Net	2	3
Total other intangible assets
Accumulated Amortization	(8)	(8)
SOUTHERN POWER CO
Other intangible assets subject to amortization:
Accumulated Amortization	(84)	(69)
Total other intangible assets
Accumulated Amortization	(84)	(69)
SOUTHERN POWER CO | PPA fair value adjustments
Other intangible assets subject to amortization:
Gross Carrying Amount	390	390
Accumulated Amortization	(84)	(69)
Other Intangible Assets, Net	306	321
Total other intangible assets
Accumulated Amortization	(84)	(69)
SOUTHERN Co GAS
Other intangible assets subject to amortization:
Gross Carrying Amount	246	246
Accumulated Amortization	(191)	(176)
Other Intangible Assets, Net	55	70
Total other intangible assets
Accumulated Amortization	(191)	(176)
Other Intangible Assets, Net	55	70
SOUTHERN Co GAS | Customer relationships
Other intangible assets subject to amortization:
Gross Carrying Amount	156	156
Accumulated Amortization	(115)	(104)
Other Intangible Assets, Net	41	52
Total other intangible assets
Accumulated Amortization	(115)	(104)
SOUTHERN Co GAS | Trade names
Other intangible assets subject to amortization:
Gross Carrying Amount	26	26
Accumulated Amortization	(12)	(10)
Other Intangible Assets, Net	14	16
Total other intangible assets
Accumulated Amortization	(12)	(10)
SOUTHERN Co GAS | Storage and transportation contracts
Other intangible assets subject to amortization:
Gross Carrying Amount	64	64
Accumulated Amortization	(64)	(62)
Other Intangible Assets, Net	0	2
Total other intangible assets
Accumulated Amortization	$ (64)	$ (62)