REGULATORY MATTERS - APC Certificate of Convenience and Necessity and RGC (Details) $ in Millions		3 Months Ended	12 Months Ended
	Sep. 23, 2021 USD ($) MW	Mar. 31, 2024 agreement	Dec. 31, 2021 USD ($) project agreement MW	Dec. 31, 2020 USD ($)	Aug. 31, 2020 USD ($) MW	Dec. 31, 2019 MW
Schedule of Regulatory Assets and Liabilities [Line Items]
Construction work in progress | $			$ 8,771	$ 8,697
Alabama Power
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved additional generating capacity, demand-side management and distributed energy resource programs					200
Additional generating capacity, petitioned megawatts, total						2,400
Construction work in progress | $			$ 1,147	$ 866
Renewable generation certificate, authorized generating capacity (in MWs)			500
Renewable generation certificate, generating capacity (in MWs)			250
Renewable generation certificate, number of projects approved | project			5
Renewable generation certificate, number of projects | project			2
Renewable generation certificate, generating capacity owned by parent (in MWs)			18
Number of purchase power agreements | agreement			3
Alabama Power | Scenario, Forecast
Schedule of Regulatory Assets and Liabilities [Line Items]
Number of purchase power agreements commencing in period | agreement		2
Alabama Power | Calhoun Power Company, LLC
Schedule of Regulatory Assets and Liabilities [Line Items]
Approximate nameplate capacity (in MWs)	743
Consideration transferred | $	$ 180
Alabama Power | Year 2020
Schedule of Regulatory Assets and Liabilities [Line Items]
PPA, additional generating capacity (in MWs)					240
Alabama Power | Years 2015
Schedule of Regulatory Assets and Liabilities [Line Items]
PPA, additional generating capacity (in MWs)			400
Plant Barry Unit 8 | Alabama Power
Schedule of Regulatory Assets and Liabilities [Line Items]
Approximate nameplate capacity (in MWs)					720
Costs incurred, percentage of estimated in-service cost					5.00%
Estimated in-service cost | $					$ 652
Construction work in progress | $			$ 304