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Derivatives - Schedule of Interest Rate Derivatives (Details) - Interest rate derivatives
$ in Millions
 9 Months Ended
Sep. 30, 2025
USD ($)
Derivative [Line Items]  
Notional Amount $ 2,465
Fair Value Gain (Loss) (184)
Southern Company parent | Fair Value Hedges of Existing Debt | Date One  
Derivative [Line Items]  
Notional Amount $ 400
Weighted Average Interest Rate Paid 0.80%
Interest Rate Received 1.75%
Fair Value Gain (Loss) $ (23)
Southern Company parent | Fair Value Hedges of Existing Debt | Date Two  
Derivative [Line Items]  
Notional Amount $ 1,000
Weighted Average Interest Rate Paid 2.48%
Interest Rate Received 3.70%
Fair Value Gain (Loss) $ (108)
Southern Company parent | Fair Value Hedges of Existing Debt | Date Three  
Derivative [Line Items]  
Notional Amount $ 565
Weighted Average Interest Rate Paid 1.56%
Interest Rate Received 6.50%
Fair Value Gain (Loss) $ 3
SOUTHERN Co GAS | Fair Value Hedges of Existing Debt  
Derivative [Line Items]  
Fair Value Gain (Loss) (56)
SOUTHERN Co GAS | Fair Value Hedges of Existing Debt | Date One  
Derivative [Line Items]  
Notional Amount $ 500
Weighted Average Interest Rate Paid 0.49%
Interest Rate Received 1.75%