SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Other Intangible Assets (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) business_unit	Dec. 31, 2018 USD ($)
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 741	$ 773
Accumulated Amortization	(280)	(235)
Other Intangible Assets, Net	461	538
Intangible assets, gross	816	848
Intangible assets, net	536	613
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	223
Accumulated Amortization	(116)	(94)
Other Intangible Assets, Net	96	129
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	64	70
Accumulated Amortization	(25)	(21)
Other Intangible Assets, Net	39	49
Storage and transportation contracts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(62)	(54)
Other Intangible Assets, Net	2	10
PPA fair value adjustments(b)
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	390	405
Accumulated Amortization	(69)	(61)
Other Intangible Assets, Net	321	344
Other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	11	11
Accumulated Amortization	(8)	(5)
Other Intangible Assets, Net	3	6
Southern Power
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	(69)	(61)
Southern Power | PPA fair value adjustments(b)
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	390	405
Accumulated Amortization	(69)	(61)
Other Intangible Assets, Net	321	344
Southern Company Gas
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	246	246
Accumulated Amortization	(176)	(145)
Other Intangible Assets, Net	70	101
Intangible assets, net	70	101
Southern Company Gas | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	156	156
Accumulated Amortization	(104)	(84)
Other Intangible Assets, Net	52	72
Southern Company Gas | Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	26	26
Accumulated Amortization	(10)	(7)
Other Intangible Assets, Net	16	19
Southern Company Gas | Storage and transportation contracts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	64	64
Accumulated Amortization	(62)	(54)
Other Intangible Assets, Net	$ 2	$ 10
PowerSecure International, Inc.
Finite-Lived Intangible Assets [Line Items]
Number of business units sold | business_unit	2