Derivatives - Pre-Tax Effects of Unrealized Derivative Gains (Losses) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	$ 277	$ 193
Energy-related derivatives
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(40)	(44)
Energy-related derivatives | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(19)	(34)
Energy-related derivatives | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(16)	(18)
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)	1	7
Energy-related derivatives | Other regulatory liabilities, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		1
Alabama Power | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(6)	(6)
Alabama Power | Energy-related derivatives | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(3)	(4)
Alabama Power | Energy-related derivatives | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(3)	(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)	0	1
Alabama Power | Energy-related derivatives | Other regulatory liabilities, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		0
Georgia Power | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(15)	(13)
Georgia Power | Energy-related derivatives | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(6)	(7)
Georgia Power | Energy-related derivatives | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(9)	(6)
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
Georgia Power | Energy-related derivatives | Other regulatory liabilities, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		0
Mississippi Power | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(6)	(7)
Mississippi Power | Energy-related derivatives | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(2)	(5)
Mississippi Power | Energy-related derivatives | Other regulatory assets, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(4)	(2)
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
Mississippi Power | Energy-related derivatives | Other regulatory liabilities, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		0
Southern Company Gas
Derivatives, Fair Value [Line Items]
Collateral already posted, aggregate fair value	277	193
Southern Company Gas | Energy-related derivatives
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(7)	3
Collateral already posted, aggregate fair value		6
Southern Company Gas | Energy-related derivatives | Other regulatory assets, current
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)	(8)	(4)
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)	$ 1	7
Southern Company Gas | Energy-related derivatives | Other regulatory liabilities, deferred
Derivatives, Fair Value [Line Items]
Total energy-related derivative gains (losses)		$ 0