CONTINGENCIES - Other Matters (Details) - USD ($) $ in Millions				1 Months Ended	3 Months Ended					9 Months Ended		12 Months Ended
	Apr. 24, 2020	Apr. 23, 2020	Feb. 20, 2020	Dec. 31, 2019	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Loss Contingencies [Line Items]
Leveraged lease investments impairment loss						$ 0	$ 154	$ 0		$ 154	$ 0
Leveraged lease investments impairment loss, net of tax							$ 74
Liabilities settled										315
Liabilities incurred										15
MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Firm transportation agreement, term				15 years
MISSISSIPPI POWER CO | Kemper IGCC
Loss Contingencies [Line Items]
Liabilities settled										2
Government grants received									$ 387
MISSISSIPPI POWER CO | Scenario, Forecast | Kemper IGCC
Loss Contingencies [Line Items]
Liabilities incurred					$ 3
GULF POWER CO | Plant Daniel Units 1 And 2
Loss Contingencies [Line Items]
Undivided ownership interest to be restructured		50.00%
Undivided ownership interest, ownership interest in individual unit	100.00%	100.00%
Minimum | MISSISSIPPI POWER CO | Scenario, Forecast | Kemper IGCC
Loss Contingencies [Line Items]
Liabilities incurred												$ 10	$ 10	$ 10	$ 10	$ 10
Maximum | MISSISSIPPI POWER CO | Scenario, Forecast | Kemper IGCC
Loss Contingencies [Line Items]
Liabilities incurred												$ 15	$ 15	$ 15	$ 15	$ 15
PennEast Pipelines
Loss Contingencies [Line Items]
Construction contingency, extension request, term			2 years
PennEast Pipelines | SOUTHERN Co GAS
Loss Contingencies [Line Items]
Construction contingency estimate						$ 300				$ 300