SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Storm Damage and Reliability Reserves (Details) - USD ($) $ in Millions	12 Months Ended			36 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2022
Liability for Catastrophe Claims [Line Items]
Reliability reserve, amount deferred	$ 191.0			$ 191.0
Alabama Power
Liability for Catastrophe Claims [Line Items]
Additional accruals authorized by state PSCs		$ 65.0	$ 100.0
Reliability reserve, amount deferred	166.0			166.0
Alabama Power | Storm damage reserves
Liability for Catastrophe Claims [Line Items]
Charges against storm damage reserves	24.0			24.0
Georgia Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves				213.0
Georgia Power | Storm damage reserves
Liability for Catastrophe Claims [Line Items]
Charges against storm damage reserves	82.0			82.0
Mississippi Power
Liability for Catastrophe Claims [Line Items]
Reliability reserve, amount deferred	25.0			25.0
Mississippi Power | Storm damage reserves
Liability for Catastrophe Claims [Line Items]
Charges against storm damage reserves	0.0			$ 0.0
Major storms
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	239.0	286.0	326.0
Major storms | Alabama Power
Liability for Catastrophe Claims [Line Items]
Additional accruals authorized by state PSCs		65.0	100.0
Storm damage reserves	19.0	75.0	112.0
Major storms | Georgia Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	213.0	213.0	213.0
Major storms | Mississippi Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	$ 6.9	$ (1.8)	$ 0.8