SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Storm Damage Reserves (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Alabama Power
Liability for Catastrophe Claims [Line Items]
Additional accruals authorized by state PSCs	$ 65.0	$ 100.0	$ 84.0
Georgia Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	213.0
Major storms
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	286.0	326.0	170.0
Major storms | Alabama Power
Liability for Catastrophe Claims [Line Items]
Additional accruals authorized by state PSCs	65.0	100.0	84.0
Storm damage reserves	75.0	112.0	139.0
Major storms | Georgia Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves		213.0	30.0
Major storms | Mississippi Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	$ (1.8)	0.8	$ 1.4
Accumulated Deferred Income Taxes | Major storms | Alabama Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves		$ 39.0