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Derivatives - Narrative (Details)
MMBTU in Millions
 3 Months Ended
Mar. 31, 2026
USD ($)
MMBTU
Mar. 31, 2025
USD ($)
Dec. 31, 2025
USD ($)
Derivative [Line Items]      
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU 13.0    
Energy-related derivative hedge gain (loss) to be reclassified within twelve months $ 0    
Gain (Loss) From Derivatives Recognized in OCI (3,000,000) $ 20,000,000  
Collateral already posted, assets, aggregate fair value 22,000,000    
Derivatives 326,000,000   $ 267,000,000
Interest rate derivatives      
Derivative [Line Items]      
Derivatives 26,000,000    
Interest rate derivatives | Cash Flow Hedging      
Derivative [Line Items]      
Gain (Loss) From Derivatives Recognized in OCI 0 (2,000,000)  
Interest rate derivatives | Cash Flow Hedging | Interest Expense      
Derivative [Line Items]      
Gain (loss) on hedges (2,000,000) (2,000,000)  
Interest rate derivatives | Fair Value Hedging | Interest Expense      
Derivative [Line Items]      
Gain (loss) on hedges 20,000,000 40,000,000  
Energy-related derivatives | Cash Flow Hedging      
Derivative [Line Items]      
Gain (Loss) From Derivatives Recognized in OCI 10,000,000 19,000,000  
Foreign currency derivatives | Cash Flow Hedging      
Derivative [Line Items]      
Gain (Loss) From Derivatives Recognized in OCI (12,000,000) 19,000,000  
Foreign currency derivatives | Cash Flow Hedging | Interest Expense      
Derivative [Line Items]      
Gain (loss) on hedges $ (3,000,000) (3,000,000)  
ALABAMA POWER CO      
Derivative [Line Items]      
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU 3.0    
Collateral already posted, assets, aggregate fair value $ 0    
Derivatives 25,000,000   18,000,000
ALABAMA POWER CO | Interest rate derivatives      
Derivative [Line Items]      
Cash flow hedge gain (loss) to be reclassified within twelve months $ 0    
GEORGIA POWER CO      
Derivative [Line Items]      
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU 5.0    
Derivatives $ 28,000,000   19,000,000
GEORGIA POWER CO | Interest rate derivatives      
Derivative [Line Items]      
Cash flow hedge gain (loss) to be reclassified within twelve months 0    
GEORGIA POWER CO | Interest rate derivatives | Cash Flow Hedging      
Derivative [Line Items]      
Gain (Loss) From Derivatives Recognized in OCI $ 0 (2,000,000)  
MISSISSIPPI POWER CO      
Derivative [Line Items]      
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU 2.0    
Derivatives $ 20,000,000   14,000,000
MISSISSIPPI POWER CO | Interest rate derivatives      
Derivative [Line Items]      
Cash flow hedge gain (loss) to be reclassified within twelve months $ 0    
SOUTHERN POWER CO      
Derivative [Line Items]      
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU 3.0    
Energy-related derivative hedge gain (loss) to be reclassified within twelve months $ 0    
Foreign currency cash flow hedge gain (loss) to be reclassified during next 12 months 0    
Gain (Loss) From Derivatives Recognized in OCI (10,000,000) 22,000,000  
Collateral already posted, assets, aggregate fair value 0    
Derivatives 2,000,000   1,000,000
SOUTHERN POWER CO | Interest rate derivatives      
Derivative [Line Items]      
Cash flow hedge gain (loss) to be reclassified within twelve months 0    
Derivatives 0    
SOUTHERN POWER CO | Energy-related derivatives      
Derivative [Line Items]      
Derivatives 0    
SOUTHERN POWER CO | Energy-related derivatives | Cash Flow Hedging      
Derivative [Line Items]      
Gain (Loss) From Derivatives Recognized in OCI 2,000,000 3,000,000  
SOUTHERN POWER CO | Foreign currency derivatives | Cash Flow Hedging      
Derivative [Line Items]      
Gain (Loss) From Derivatives Recognized in OCI (12,000,000) 19,000,000  
SOUTHERN POWER CO | Foreign currency derivatives | Cash Flow Hedging | Interest Expense      
Derivative [Line Items]      
Gain (loss) on hedges (3,000,000) (3,000,000)  
Registrants | Derivative Counterparties      
Derivative [Line Items]      
Collateral already posted, assets, aggregate fair value 0    
SOUTHERN Co GAS      
Derivative [Line Items]      
Energy-related derivative hedge gain (loss) to be reclassified within twelve months 0    
Collateral already posted, assets, aggregate fair value 22,000,000   33,000,000
Derivatives 65,000,000   $ 65,000,000
SOUTHERN Co GAS | Interest rate derivatives      
Derivative [Line Items]      
Cash flow hedge gain (loss) to be reclassified within twelve months 0    
SOUTHERN Co GAS | Interest rate derivatives | Fair Value Hedging | Interest Expense      
Derivative [Line Items]      
Gain (loss) on hedges 29,000,000 17,000,000  
SOUTHERN Co GAS | Energy-related derivatives | Cash Flow Hedging      
Derivative [Line Items]      
Gain (Loss) From Derivatives Recognized in OCI 8,000,000 $ 16,000,000  
Traditional Electric Operating Companies | Energy-related derivatives      
Derivative [Line Items]      
Derivatives $ 0