DERIVATIVES - Pre-Tax Effects of Unrealized Derivative Gains (Losses) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	$ 99	$ 277
Energy-related derivatives
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(94)	(40)
Energy-related derivatives | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(63)	(19)
Energy-related derivatives | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(37)	(16)
Energy-related derivatives | Assets held for sale, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		(6)
Energy-related derivatives | Other regulatory liabilities, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	6	1
Alabama Power | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(20)	(6)
Alabama Power | Energy-related derivatives | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(14)	(3)
Alabama Power | Energy-related derivatives | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(8)	(3)
Alabama Power | Energy-related derivatives | Assets held for sale, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		0
Alabama Power | Energy-related derivatives | Other regulatory liabilities, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	2	0
Georgia Power | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(49)	(15)
Georgia Power | Energy-related derivatives | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(31)	(6)
Georgia Power | Energy-related derivatives | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(18)	(9)
Georgia Power | Energy-related derivatives | Assets held for sale, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		0
Georgia Power | Energy-related derivatives | Other regulatory liabilities, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	0	0
Mississippi Power | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(26)	(6)
Mississippi Power | Energy-related derivatives | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(15)	(2)
Mississippi Power | Energy-related derivatives | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(11)	(4)
Mississippi Power | Energy-related derivatives | Assets held for sale, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		0
Mississippi Power | Energy-related derivatives | Other regulatory liabilities, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	0	0
Southern Company Gas
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	99	277
Southern Company Gas | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	1	(7)
Southern Company Gas | Energy-related derivatives | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(3)	(8)
Southern Company Gas | Energy-related derivatives | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	0	0
Southern Company Gas | Energy-related derivatives | Assets held for sale, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		0
Southern Company Gas | Energy-related derivatives | Other regulatory liabilities, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	$ 4	$ 1