DERIVATIVES - Schedule of Unrealized Gain (Loss) Recognized in the Balance Sheet (Details) - Energy-related derivatives - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	$ 12	$ (94)
Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		(63)
Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(2)	(37)
Other regulatory liabilities, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	12	6
Other regulatory liabilities, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	2
Alabama Power
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	6	(20)
Alabama Power | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		(14)
Alabama Power | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	0	(8)
Alabama Power | Other regulatory liabilities, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	5	2
Alabama Power | Other regulatory liabilities, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	1
Georgia Power
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	2	(49)
Georgia Power | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		(31)
Georgia Power | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(1)	(18)
Georgia Power | Other regulatory liabilities, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	2	0
Georgia Power | Other regulatory liabilities, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	1
Mississippi Power
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	0	(26)
Mississippi Power | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		(15)
Mississippi Power | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(1)	(11)
Mississippi Power | Other regulatory liabilities, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	1	0
Mississippi Power | Other regulatory liabilities, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	0
Southern Company Gas
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	4	1
Southern Company Gas | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		(3)
Southern Company Gas | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	0	0
Southern Company Gas | Other regulatory liabilities, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	4	$ 4
Southern Company Gas | Other regulatory liabilities, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	$ 0