SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Storm Damage Reserves (Details) - Major storms - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	$ 326,000,000	$ 170,000,000	$ 74,000,000
Alabama Power
Liability for Catastrophe Claims [Line Items]
Additional accruals authorized by state PSCs	100,000,000	84,000,000	0
Storm damage reserves	112,000,000	139,000,000	16,000,000
Georgia Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	213,000,000	30,000,000	30,000,000
Mississippi Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves	$ 1,000,000	1,000,000	1,000,000
Gulf Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves			$ 26,900,000
Accumulated Deferred Income Taxes | Alabama Power
Liability for Catastrophe Claims [Line Items]
Storm damage reserves		$ 39,000,000