Derivatives - Pre-tax Effects of Derivatives Designated as Cash Flow Hedging Instruments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Collateral already posted, aggregate fair value	$ 190
Gain (Loss) Recognized in OCI on Derivative	(39)	$ 63
SOUTHERN POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(39)	64
SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Collateral already posted, aggregate fair value	190		$ 277
Energy-related derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(9)		(40)
Gain (Loss) Recognized in OCI on Derivative	0	12
Energy-related derivatives | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(5)		(19)
Energy-related derivatives | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(11)		(16)
Energy-related derivatives | Assets held for sale, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)			(6)
Energy-related derivatives | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	7		1
Energy-related derivatives | ALABAMA POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(2)		(6)
Energy-related derivatives | ALABAMA POWER CO | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(1)		(3)
Energy-related derivatives | ALABAMA POWER CO | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(2)		(3)
Energy-related derivatives | ALABAMA POWER CO | Assets held for sale, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)			0
Energy-related derivatives | ALABAMA POWER CO | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	1		0
Energy-related derivatives | GEORGIA POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(7)		(15)
Energy-related derivatives | GEORGIA POWER CO | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(2)		(6)
Energy-related derivatives | GEORGIA POWER CO | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(6)		(9)
Energy-related derivatives | GEORGIA POWER CO | Assets held for sale, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)			0
Energy-related derivatives | GEORGIA POWER CO | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	1		0
Energy-related derivatives | MISSISSIPPI POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(3)		(6)
Energy-related derivatives | MISSISSIPPI POWER CO | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(1)		(2)
Energy-related derivatives | MISSISSIPPI POWER CO | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(3)		(4)
Energy-related derivatives | MISSISSIPPI POWER CO | Assets held for sale, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)			0
Energy-related derivatives | MISSISSIPPI POWER CO | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	1		0
Energy-related derivatives | SOUTHERN POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	0	11
Energy-related derivatives | SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	3		(7)
Collateral already posted, aggregate fair value	2
Energy-related derivatives | SOUTHERN Co GAS | Other regulatory assets, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(1)		(8)
Energy-related derivatives | SOUTHERN Co GAS | Other regulatory assets, deferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0		0
Energy-related derivatives | SOUTHERN Co GAS | Assets held for sale, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)			0
Energy-related derivatives | SOUTHERN Co GAS | Other regulatory liabilities, current
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	4		$ 1
Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	0	(2)
Foreign currency derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	(39)	53
Foreign currency derivatives | SOUTHERN POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivative	$ (39)	$ 53