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RETIREMENT BENEFITS - Components of Net Periodic Benefit Cost and Estimated Future Benefit Payments (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Pension plans |  |  |
Components of net periodic |  |  |
Service cost | $ 275 | $ 412 | $ 434
Interest cost | 626 | 408 | 346
Expected return on plan assets | (1,229) | (1,265) | (1,191)
Recognized net (gain) loss | 32 | 240 | 314
Net amortization | 0 | 0 | 1
Prior service cost | 0 | 0 | 0
Total change | (296) | (205) | (96)
Benefit payments: |  |  |
2024 | 748 |  |
2025 | 776 |  |
2026 | 801 |  |
2027 | 822 |  |
2028 | 840 |  |
2029 to 2033 | 4,417 |  |
Other Postretirement Benefits Plan |  |  |
Components of net periodic |  |  |
Service cost | 15 | 23 | 24
Interest cost | 70 | 42 | 35
Expected return on plan assets | (83) | (80) | (76)
Prior service cost | (11) | (1) | 2
Total change | (9) | (16) | (15)
Benefit payments: |  |  |
2024 | 111 |  |
2025 | 109 |  |
2026 | 108 |  |
2027 | 108 |  |
2028 | 108 |  |
2029 to 2033 | 523 |  |
Alabama Power | Pension plans |  |  |
Components of net periodic |  |  |
Service cost | 64 | 99 | 102
Interest cost | 145 | 96 | 82
Expected return on plan assets | (297) | (306) | (287)
Recognized net (gain) loss | 9 | 62 | 82
Net amortization | 1 | 1 | 1
Prior service cost | 0 | 0 | 0
Total change | (78) | (48) | (20)
Benefit payments: |  |  |
2024 | 164 |  |
2025 | 171 |  |
2026 | 177 |  |
2027 | 182 |  |
2028 | 187 |  |
2029 to 2033 | 988 |  |
Alabama Power | Other Postretirement Benefits Plan |  |  |
Components of net periodic |  |  |
Service cost | 4 | 6 | 6
Interest cost | 17 | 10 | 9
Expected return on plan assets | (33) | (32) | (30)
Prior service cost | (3) | 0 | 0
Total change | (15) | (16) | (15)
Benefit payments: |  |  |
2024 | 24 |  |
2025 | 24 |  |
2026 | 24 |  |
2027 | 24 |  |
2028 | 25 |  |
2029 to 2033 | 123 |  |
Georgia Power | Pension plans |  |  |
Components of net periodic |  |  |
Service cost | 68 | 103 | 112
Interest cost | 191 | 123 | 104
Expected return on plan assets | (385) | (399) | (375)
Recognized net (gain) loss | 13 | 75 | 100
Net amortization | 1 | 1 | 1
Prior service cost | 0 | 0 | 0
Total change | (112) | (97) | (58)
Benefit payments: |  |  |
2024 | 241 |  |
2025 | 248 |  |
2026 | 254 |  |
2027 | 257 |  |
2028 | 260 |  |
2029 to 2033 | 1,337 |  |
Georgia Power | Other Postretirement Benefits Plan |  |  |
Components of net periodic |  |  |
Service cost | 4 | 6 | 7
Interest cost | 25 | 15 | 12
Expected return on plan assets | (29) | (28) | (26)
Prior service cost | (3) | 2 | 2
Total change | (3) | (5) | (5)
Benefit payments: |  |  |
2024 | 40 |  |
2025 | 39 |  |
2026 | 38 |  |
2027 | 38 |  |
2028 | 38 |  |
2029 to 2033 | 188 |  |
Mississippi Power | Pension plans |  |  |
Components of net periodic |  |  |
Service cost | 11 | 17 | 18
Interest cost | 28 | 18 | 16
Expected return on plan assets | (56) | (57) | (55)
Recognized net (gain) loss | 2 | 11 | 15
Net amortization | 0 | 0 | 0
Prior service cost | 0 | 0 | 0
Total change | (15) | (11) | (6)
Benefit payments: |  |  |
2024 | 33 |  |
2025 | 34 |  |
2026 | 36 |  |
2027 | 37 |  |
2028 | 38 |  |
2029 to 2033 | 201 |  |
Mississippi Power | Other Postretirement Benefits Plan |  |  |
Components of net periodic |  |  |
Service cost | 1 | 1 | 1
Interest cost | 3 | 2 | 1
Expected return on plan assets | (3) | (2) | (1)
Prior service cost | 0 | 0 | 0
Total change | 1 | 1 | 1
Benefit payments: |  |  |
2024 | 5 |  |
2025 | 5 |  |
2026 | 5 |  |
2027 | 5 |  |
2028 | 4 |  |
2029 to 2033 | 21 |  |
Southern Power | Pension plans |  |  |
Components of net periodic |  |  |
Service cost | 6 | 9 | 10
Interest cost | 8 | 6 | 5
Expected return on plan assets | (15) | (15) | (14)
Recognized net (gain) loss | 0 | 2 | 3
Net amortization | 0 | 0 | 0
Prior service cost | 0 | 0 | 0
Total change | (1) | 2 | 4
Benefit payments: |  |  |
2024 | 7 |  |
2025 | 7 |  |
2026 | 8 |  |
2027 | 8 |  |
2028 | 7 |  |
2029 to 2033 | 47 |  |
Southern Power | Other Postretirement Benefits Plan |  |  |
Components of net periodic |  |  |
Service cost | 0 | 0 | 0
Interest cost | 0 | 0 | 0
Expected return on plan assets | 1 | 1 | 1
Prior service cost | 0 | 0 | 0
Total change | 1 | 1 | 1
Benefit payments: |  |  |
2024 | 1 |  |
2025 | 1 |  |
2026 | 1 |  |
2027 | 1 |  |
2028 | 1 |  |
2029 to 2033 | 1 |  |
Southern Company Gas | Pension plans |  |  |
Components of net periodic |  |  |
Service cost | 24 | 34 | 37
Interest cost | 42 | 28 | 24
Expected return on plan assets | (85) | (91) | (86)
Recognized net (gain) loss | (5) | 8 | 13
Net amortization | 15 | 15 | 15
Prior service cost | (3) | (3) | (3)
Total change | (12) | (9) | 0
Benefit payments: |  |  |
2024 | 57 |  |
2025 | 59 |  |
2026 | 60 |  |
2027 | 62 |  |
2028 | 64 |  |
2029 to 2033 | 336 |  |
Southern Company Gas | Other Postretirement Benefits Plan |  |  |
Components of net periodic |  |  |
Service cost | 1 | 1 | 2
Interest cost | 9 | 5 | 4
Expected return on plan assets | (10) | (9) | (10)
Prior service cost | 6 | 6 | 6
Total change | 6 | $ 3 | $ 2
Benefit payments: |  |  |
2024 | 17 |  |
2025 | 17 |  |
2026 | 16 |  |
2027 | 16 |  |
2028 | 15 |  |
2029 to 2033 | $ 64 |  |

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