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Derivatives - Narrative (Details)
MMBTU in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
USD ($)
Sep. 30, 2024
USD ($)
Sep. 30, 2025
USD ($)
MMBTU
Sep. 30, 2024
USD ($)
Dec. 31, 2024
USD ($)
Derivative [Line Items]          
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU     1.5    
Energy-related derivative hedge gain (loss) to be reclassified within twelve months $ 0   $ 0    
Gain (Loss) Recognized in OCI on Derivatives (18,000,000) $ (2,000,000) 48,000,000 $ (7,000,000)  
Collateral already posted, assets, aggregate fair value 10,000,000   10,000,000    
Derivatives 238,000,000   238,000,000   $ 554,000,000
Interest rate derivatives          
Derivative [Line Items]          
Derivatives 20,000,000   20,000,000    
Interest rate derivatives | Cash Flow Hedging          
Derivative [Line Items]          
Gain (Loss) Recognized in OCI on Derivatives (1,000,000) (8,000,000) 6,000,000 16,000,000  
Interest rate derivatives | Cash Flow Hedging | Interest Expense          
Derivative [Line Items]          
Gain (loss) on hedges (4,000,000) (4,000,000) (10,000,000) (12,000,000)  
Interest rate derivatives | Fair Value Hedging | Interest Expense          
Derivative [Line Items]          
Gain (loss) on hedges 16,000,000 78,000,000 85,000,000 47,000,000  
Energy-related derivatives | Energy-related derivatives not designated as hedging instruments | Cost of Natural Gas          
Derivative [Line Items]          
Derivative instruments not designated as hedging instruments, gain (loss), net (1,000,000) 6,000,000 (7,000,000) 69,000,000  
Energy-related derivatives | Cash Flow Hedging          
Derivative [Line Items]          
Gain (Loss) Recognized in OCI on Derivatives (12,000,000) (7,000,000) (6,000,000) (11,000,000)  
Foreign currency derivatives | Cash Flow Hedging          
Derivative [Line Items]          
Gain (Loss) Recognized in OCI on Derivatives (6,000,000) 15,000,000 60,000,000 (4,000,000)  
Foreign currency derivatives | Cash Flow Hedging | Interest Expense          
Derivative [Line Items]          
Gain (loss) on hedges (2,000,000) (3,000,000) $ (8,000,000) (9,000,000)  
ALABAMA POWER CO          
Derivative [Line Items]          
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU     0.4    
Collateral already posted, assets, aggregate fair value 0   $ 0    
Derivatives 8,000,000   8,000,000   23,000,000
ALABAMA POWER CO | Interest rate derivatives          
Derivative [Line Items]          
Cash flow hedge gain (loss) to be reclassified within twelve months 0   $ 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     0.6    
Derivatives 8,000,000   $ 8,000,000   27,000,000
GEORGIA POWER CO | Interest rate derivatives          
Derivative [Line Items]          
Cash flow hedge gain (loss) to be reclassified within twelve months 0   0    
GEORGIA POWER CO | Interest rate derivatives | Cash Flow Hedging          
Derivative [Line Items]          
Gain (Loss) Recognized in OCI on Derivatives 0 1,000,000 $ 3,000,000 17,000,000  
MISSISSIPPI POWER CO          
Derivative [Line Items]          
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU     0.2    
Derivatives 8,000,000   $ 8,000,000   17,000,000
MISSISSIPPI POWER CO | Interest rate derivatives          
Derivative [Line Items]          
Cash flow hedge gain (loss) to be reclassified within twelve months 0   0    
MISSISSIPPI POWER CO | Interest rate derivatives | Cash Flow Hedging          
Derivative [Line Items]          
Gain (Loss) Recognized in OCI on Derivatives 0 0 $ 0 7,000,000  
SOUTHERN POWER CO          
Derivative [Line Items]          
Expected volume of natural gas subject to option to sell back excess gas due to operational constraints | MMBTU     0.3    
Energy-related derivative hedge gain (loss) to be reclassified within twelve months 0   $ 0    
Foreign currency cash flow hedge gain (loss) to be reclassified during next 12 months 20,000,000   20,000,000    
Gain (Loss) Recognized in OCI on Derivatives (8,000,000) 13,000,000 60,000,000 (6,000,000)  
Collateral already posted, assets, aggregate fair value 0   0    
Derivatives 1,000,000   1,000,000   51,000,000
SOUTHERN POWER CO | Interest rate derivatives          
Derivative [Line Items]          
Cash flow hedge gain (loss) to be reclassified within twelve months 0   0    
SOUTHERN POWER CO | Energy-related derivatives          
Derivative [Line Items]          
Derivatives 0   0    
SOUTHERN POWER CO | Energy-related derivatives | Cash Flow Hedging          
Derivative [Line Items]          
Gain (Loss) Recognized in OCI on Derivatives (2,000,000) (2,000,000) 0 (2,000,000)  
SOUTHERN POWER CO | Foreign currency derivatives | Cash Flow Hedging          
Derivative [Line Items]          
Gain (Loss) Recognized in OCI on Derivatives (6,000,000) 15,000,000 60,000,000 (4,000,000)  
SOUTHERN POWER CO | Foreign currency derivatives | Cash Flow Hedging | Interest Expense          
Derivative [Line Items]          
Gain (loss) on hedges (2,000,000) (3,000,000) (8,000,000) (9,000,000)  
Registrants | Derivative Counterparties          
Derivative [Line Items]          
Collateral already posted, assets, aggregate fair value 0   0    
SOUTHERN Co GAS          
Derivative [Line Items]          
Energy-related derivative hedge gain (loss) to be reclassified within twelve months 0   0    
Gain (Loss) Recognized in OCI on Derivatives (11,000,000) (10,000,000) (3,000,000) (14,000,000)  
Collateral already posted, assets, aggregate fair value 10,000,000   10,000,000   17,000,000
Derivatives 57,000,000   57,000,000   $ 84,000,000
SOUTHERN Co GAS | Interest rate derivatives          
Derivative [Line Items]          
Cash flow hedge gain (loss) to be reclassified within twelve months 0   0    
SOUTHERN Co GAS | Interest rate derivatives | Cash Flow Hedging          
Derivative [Line Items]          
Gain (Loss) Recognized in OCI on Derivatives (1,000,000) (6,000,000) 3,000,000 (5,000,000)  
SOUTHERN Co GAS | Interest rate derivatives | Cash Flow Hedging | Interest Expense          
Derivative [Line Items]          
Gain (loss) on hedges 0 0 (1,000,000) 0  
SOUTHERN Co GAS | Interest rate derivatives | Fair Value Hedging | Interest Expense          
Derivative [Line Items]          
Gain (loss) on hedges 7,000,000 28,000,000 28,000,000 18,000,000  
SOUTHERN Co GAS | Energy-related derivatives | Cash Flow Hedging          
Derivative [Line Items]          
Gain (Loss) Recognized in OCI on Derivatives (10,000,000) $ (4,000,000) (6,000,000) $ (9,000,000)  
Traditional Electric Operating Companies | Energy-related derivatives          
Derivative [Line Items]          
Derivatives $ 0   $ 0