Derivatives - Schedule of Pre-tax Effects of Unrealized Derivative Gains (Losses) (Details) - Energy-related derivatives - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	$ (91)	$ (45)
Balance Sheet Location [Axis]: so:OtherRegulatoryAssetsCurrent
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(75)	(48)
Balance Sheet Location [Axis]: so:OtherRegulatoryAssetsDeferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(31)	(8)
Balance Sheet Location [Axis]: so:OtherRegulatoryLiabilitiesCurrent
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	15	7
Balance Sheet Location [Axis]: so:OtherRegulatoryLiabilitiesDeferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)		4
ALABAMA POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(29)	(12)
ALABAMA POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryAssetsCurrent
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(26)	(13)
ALABAMA POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryAssetsDeferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(13)	(5)
ALABAMA POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryLiabilitiesCurrent
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	10	4
ALABAMA POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryLiabilitiesDeferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)		2
GEORGIA POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(37)	(16)
GEORGIA POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryAssetsCurrent
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(27)	(17)
GEORGIA POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryAssetsDeferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(10)	(1)
GEORGIA POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryLiabilitiesCurrent
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0	1
GEORGIA POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryLiabilitiesDeferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)		1
MISSISSIPPI POWER CO
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(26)	(13)
MISSISSIPPI POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryAssetsCurrent
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(18)	(12)
MISSISSIPPI POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryAssetsDeferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(8)	(2)
MISSISSIPPI POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryLiabilitiesCurrent
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0	0
MISSISSIPPI POWER CO \| Balance Sheet Location [Axis]: so:OtherRegulatoryLiabilitiesDeferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)		1
SOUTHERN Co GAS
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	1	(4)
SOUTHERN Co GAS \| Balance Sheet Location [Axis]: so:OtherRegulatoryAssetsCurrent
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	(4)	(6)
SOUTHERN Co GAS \| Balance Sheet Location [Axis]: so:OtherRegulatoryAssetsDeferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	0	0
SOUTHERN Co GAS \| Balance Sheet Location [Axis]: so:OtherRegulatoryLiabilitiesCurrent
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)	$ 5	2
SOUTHERN Co GAS \| Balance Sheet Location [Axis]: so:OtherRegulatoryLiabilitiesDeferred
Derivative Instruments, Gain (Loss) [Line Items]
Total energy-related derivative gains (losses)		$ 0