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Retirement Benefits (Details) - USD ($)
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Dec. 31, 2026
Pension Plans      
Pension Plans and Postretirement Plans      
Service cost $ 69,000,000 $ 66,000,000  
Interest cost 162,000,000 166,000,000  
Expected return on plan assets (322,000,000) (320,000,000)  
Amortization:      
Prior service costs 0 0  
Regulatory asset 0 0  
Net gain 18,000,000 9,000,000  
Net periodic pension income (73,000,000) (79,000,000)  
Pension Plans | Scenario, Forecast      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, expected future employer contributions, remainder of fiscal year     $ 0
Pension Plans | ALABAMA POWER CO      
Pension Plans and Postretirement Plans      
Service cost 15,000,000 15,000,000  
Interest cost 38,000,000 39,000,000  
Expected return on plan assets (79,000,000) (78,000,000)  
Amortization:      
Prior service costs 0 0  
Regulatory asset 0 0  
Net gain 5,000,000 2,000,000  
Net periodic pension income (21,000,000) (22,000,000)  
Pension Plans | GEORGIA POWER CO      
Pension Plans and Postretirement Plans      
Service cost 17,000,000 16,000,000  
Interest cost 47,000,000 49,000,000  
Expected return on plan assets (99,000,000) (99,000,000)  
Amortization:      
Prior service costs 0 0  
Regulatory asset 0 0  
Net gain 6,000,000 4,000,000  
Net periodic pension income (29,000,000) (30,000,000)  
Pension Plans | MISSISSIPPI POWER CO      
Pension Plans and Postretirement Plans      
Service cost 3,000,000 3,000,000  
Interest cost 7,000,000 7,000,000  
Expected return on plan assets (15,000,000) (15,000,000)  
Amortization:      
Prior service costs 0 0  
Regulatory asset 0 0  
Net gain 1,000,000 1,000,000  
Net periodic pension income (4,000,000) (4,000,000)  
Pension Plans | SOUTHERN POWER CO      
Pension Plans and Postretirement Plans      
Service cost 2,000,000 1,000,000  
Interest cost 2,000,000 2,000,000  
Expected return on plan assets (4,000,000) (4,000,000)  
Amortization:      
Prior service costs 0 0  
Regulatory asset 0 0  
Net gain 0 1,000,000  
Net periodic pension income 0 0  
Pension Plans | SOUTHERN Co GAS      
Pension Plans and Postretirement Plans      
Service cost 7,000,000 7,000,000  
Interest cost 11,000,000 11,000,000  
Expected return on plan assets (21,000,000) (22,000,000)  
Amortization:      
Prior service costs (1,000,000) (1,000,000)  
Regulatory asset 3,000,000 4,000,000  
Net gain 0 0  
Net periodic pension income (1,000,000) (1,000,000)  
Postretirement Benefits      
Pension Plans and Postretirement Plans      
Service cost 3,000,000 3,000,000  
Interest cost 16,000,000 17,000,000  
Expected return on plan assets (23,000,000) (23,000,000)  
Amortization:      
Prior service costs 1,000,000 1,000,000  
Regulatory asset   0  
Net gain (2,000,000) (3,000,000)  
Net periodic pension income (5,000,000) (5,000,000)  
Postretirement Benefits | ALABAMA POWER CO      
Pension Plans and Postretirement Plans      
Service cost 1,000,000 1,000,000  
Interest cost 4,000,000 4,000,000  
Expected return on plan assets (9,000,000) (9,000,000)  
Amortization:      
Prior service costs 0 0  
Regulatory asset   0  
Net gain 0 0  
Net periodic pension income (4,000,000) (4,000,000)  
Postretirement Benefits | GEORGIA POWER CO      
Pension Plans and Postretirement Plans      
Service cost 1,000,000 1,000,000  
Interest cost 6,000,000 6,000,000  
Expected return on plan assets (8,000,000) (8,000,000)  
Amortization:      
Prior service costs 0 0  
Regulatory asset   0  
Net gain 0 0  
Net periodic pension income (1,000,000) (1,000,000)  
Postretirement Benefits | MISSISSIPPI POWER CO      
Pension Plans and Postretirement Plans      
Service cost 0 0  
Interest cost 1,000,000 1,000,000  
Expected return on plan assets (1,000,000) 0  
Amortization:      
Prior service costs 0 0  
Regulatory asset   0  
Net gain 0 (1,000,000)  
Net periodic pension income 0 0  
Postretirement Benefits | SOUTHERN POWER CO      
Pension Plans and Postretirement Plans      
Service cost 0 0  
Interest cost 0 0  
Expected return on plan assets 0 0  
Amortization:      
Prior service costs 0 0  
Regulatory asset   0  
Net gain 0 0  
Net periodic pension income 0 0  
Postretirement Benefits | SOUTHERN Co GAS      
Pension Plans and Postretirement Plans      
Service cost 0 0  
Interest cost 2,000,000 2,000,000  
Expected return on plan assets (2,000,000) (2,000,000)  
Amortization:      
Prior service costs 0 0  
Regulatory asset   2,000,000  
Net gain (2,000,000) (2,000,000)  
Net periodic pension income $ (2,000,000) $ 0