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Derivatives - Schedule of Pre-Tax Effects of Cash Flow and Fair Value Hedging on Income (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Other Income (Expense)        
Derivative [Line Items]        
Amount excluded from effectiveness testing recognized in earnings $ (1) $ 3 $ 12 $ 8
Cash Flow Hedging | Energy-related derivatives | Cost of Goods and Services Sold | Fuel        
Derivative [Line Items]        
Gain (loss) on hedges 0 (3) 1 (5)
Cash Flow Hedging | Energy-related derivatives | Cost of Goods and Services Sold | Natural gas        
Derivative [Line Items]        
Gain (loss) on hedges (1) (4) (1) (34)
Cash Flow Hedging | Energy-related derivatives | Operating Expense        
Derivative [Line Items]        
Gain (loss) on hedges 0 0 0 (1)
Cash Flow Hedging | Interest rate derivatives | Interest Expense        
Derivative [Line Items]        
Gain (loss) on hedges (4) (4) (10) (12)
Cash Flow Hedging | Foreign currency derivatives | Interest Expense        
Derivative [Line Items]        
Gain (loss) on hedges (2) (3) (8) (9)
Cash Flow Hedging | Foreign currency derivatives | Other Income (Expense)        
Derivative [Line Items]        
Gain (loss) on hedges 1 24 68 7
Fair Value Hedging | Interest rate derivatives | Interest Expense        
Derivative [Line Items]        
Gain (loss) on hedges 16 78 85 47
Fair Value Hedging | Foreign currency derivatives        
Derivative [Line Items]        
Amount excluded from effectiveness testing recognized in earnings 1 (2) (12) (8)
Fair Value Hedging | Foreign currency derivatives | Other Income (Expense)        
Derivative [Line Items]        
Gain (loss) on hedges 5 58 160 79
SOUTHERN POWER CO | Cash Flow Hedging | Energy-related derivatives | Cost of Goods and Services Sold | Fuel        
Derivative [Line Items]        
Gain (loss) on hedges 0 (3) 1 (5)
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Interest Expense        
Derivative [Line Items]        
Gain (loss) on hedges (2) (3) (8) (9)
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Other Income (Expense)        
Derivative [Line Items]        
Gain (loss) on hedges 1 24 68 7
SOUTHERN Co GAS | Cash Flow Hedging | Energy-related derivatives | Operating Expense        
Derivative [Line Items]        
Gain (loss) on hedges 0 0 0 (1)
SOUTHERN Co GAS | Cash Flow Hedging | Energy-related derivatives | Cost of Sales | Natural gas        
Derivative [Line Items]        
Gain (loss) on hedges (1) (4) (1) (34)
SOUTHERN Co GAS | Cash Flow Hedging | Interest rate derivatives | Interest Expense        
Derivative [Line Items]        
Gain (loss) on hedges 0 0 (1) 0
SOUTHERN Co GAS | Fair Value Hedging | Interest rate derivatives | Interest Expense        
Derivative [Line Items]        
Gain (loss) on hedges $ 7 $ 28 $ 28 $ 18