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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
Mar. 31, 2026
Mar. 31, 2025
Other Income (Expense)    
Derivative [Line Items]    
Amount excluded from effectiveness testing recognized in earnings $ 1 $ 16
Cash Flow Hedging | Energy-related derivatives | Cost of Goods and Services Sold | Fuel    
Derivative [Line Items]    
Gain (loss) on hedges 3 1
Cash Flow Hedging | Energy-related derivatives | Cost of Goods and Services Sold | Natural gas    
Derivative [Line Items]    
Gain (loss) on hedges 10 (1)
Cash Flow Hedging | Interest rate derivatives | Interest Expense    
Derivative [Line Items]    
Gain (loss) on hedges (2) (2)
Cash Flow Hedging | Foreign currency derivatives | Interest Expense    
Derivative [Line Items]    
Gain (loss) on hedges (3) (3)
Cash Flow Hedging | Foreign currency derivatives | Other Income (Expense)    
Derivative [Line Items]    
Gain (loss) on hedges (13) 21
Fair Value Hedging | Interest rate derivatives | Interest Expense    
Derivative [Line Items]    
Gain (loss) on hedges 20 40
Fair Value Hedging | Foreign currency derivatives    
Derivative [Line Items]    
Amount excluded from effectiveness testing recognized in earnings (1) (16)
Fair Value Hedging | Foreign currency derivatives | Other Income (Expense)    
Derivative [Line Items]    
Gain (loss) on hedges (32) 40
SOUTHERN POWER CO | Cash Flow Hedging | Energy-related derivatives | Cost of Goods and Services Sold | Fuel    
Derivative [Line Items]    
Gain (loss) on hedges 3 1
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Interest Expense    
Derivative [Line Items]    
Gain (loss) on hedges (3) (3)
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Other Income (Expense)    
Derivative [Line Items]    
Gain (loss) on hedges (13) 21
SOUTHERN Co GAS | Cash Flow Hedging | Energy-related derivatives | Cost of Sales | Natural gas    
Derivative [Line Items]    
Gain (loss) on hedges 10 (1)
SOUTHERN Co GAS | Fair Value Hedging | Interest rate derivatives | Interest Expense    
Derivative [Line Items]    
Gain (loss) on hedges $ 29 $ 17