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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, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Pension plans |  |  |
Components of net periodic |  |  |
Service cost | $ 434 | $ 376 | $ 292
Interest cost | 346 | 432 | 492
Expected return on plan assets | (1,191) | (1,100) | (885)
Recognized net loss | 314 | 269 | 120
Net amortization | 1 | 1 | 2
Prior service cost | 0 | 0 | 0
Total change | (96) | (22) | 21
Benefit payments: |  |  |
2022 | 690 |  |
2023 | 714 |  |
2024 | 736 |  |
2025 | 759 |  |
2026 | 780 |  |
2027 to 2031 | 4,138 |  |
Other postretirement benefit plans |  |  |
Components of net periodic |  |  |
Service cost | 24 | 22 | 18
Interest cost | 35 | 54 | 69
Expected return on plan assets | (76) | (72) | (65)
Prior service cost | 2 | 1 | 0
Total change | (15) | 5 | 22
Benefit payments: |  |  |
2022 | 111 |  |
2023 | 110 |  |
2024 | 109 |  |
2025 | 112 |  |
2026 | 112 |  |
2027 to 2031 | 552 |  |
Alabama Power | Pension plans |  |  |
Components of net periodic |  |  |
Service cost | 102 | 89 | 69
Interest cost | 82 | 100 | 114
Expected return on plan assets | (287) | (264) | (206)
Recognized net loss | 82 | 71 | 37
Net amortization | 1 | 1 | 0
Prior service cost | 0 | 0 | 0
Total change | (20) | (3) | 14
Benefit payments: |  |  |
2022 | 148 |  |
2023 | 155 |  |
2024 | 159 |  |
2025 | 165 |  |
2026 | 171 |  |
2027 to 2031 | 921 |  |
Alabama Power | Other postretirement benefit plans |  |  |
Components of net periodic |  |  |
Service cost | 6 | 6 | 5
Interest cost | 9 | 13 | 16
Expected return on plan assets | (30) | (29) | (26)
Prior service cost | 0 | 0 | 4
Total change | (15) | (10) | (1)
Benefit payments: |  |  |
2022 | 24 |  |
2023 | 24 |  |
2024 | 24 |  |
2025 | 25 |  |
2026 | 25 |  |
2027 to 2031 | 128 |  |
Georgia Power | Pension plans |  |  |
Components of net periodic |  |  |
Service cost | 112 | 96 | 74
Interest cost | 104 | 133 | 156
Expected return on plan assets | (375) | (347) | (292)
Recognized net loss | 100 | 86 | 44
Net amortization | 1 | 1 | 1
Prior service cost | 0 | 0 | 0
Total change | (58) | (31) | (17)
Benefit payments: |  |  |
2022 | 223 |  |
2023 | 229 |  |
2024 | 235 |  |
2025 | 240 |  |
2026 | 245 |  |
2027 to 2031 | 1,275 |  |
Georgia Power | Other postretirement benefit plans |  |  |
Components of net periodic |  |  |
Service cost | 7 | 6 | 5
Interest cost | 12 | 20 | 26
Expected return on plan assets | (26) | (26) | (25)
Prior service cost | 2 | 2 | 1
Total change | (5) | 2 | 7
Benefit payments: |  |  |
2022 | 40 |  |
2023 | 39 |  |
2024 | 38 |  |
2025 | 40 |  |
2026 | 40 |  |
2027 to 2031 | 199 |  |
Mississippi Power | Pension plans |  |  |
Components of net periodic |  |  |
Service cost | 18 | 15 | 12
Interest cost | 16 | 20 | 22
Expected return on plan assets | (55) | (51) | (40)
Recognized net loss | 15 | 13 | 6
Net amortization | 0 | 0 | 0
Prior service cost | 0 | 0 | 0
Total change | (6) | (3) | 0
Benefit payments: |  |  |
2022 | 31 |  |
2023 | 31 |  |
2024 | 32 |  |
2025 | 34 |  |
2026 | 35 |  |
2027 to 2031 | 187 |  |
Mississippi Power | Other postretirement benefit plans |  |  |
Components of net periodic |  |  |
Service cost | 1 | 1 | 1
Interest cost | 1 | 2 | 3
Expected return on plan assets | (1) | (1) | (2)
Prior service cost | 0 | 0 | 0
Total change | 1 | 2 | 2
Benefit payments: |  |  |
2022 | 5 |  |
2023 | 5 |  |
2024 | 5 |  |
2025 | 5 |  |
2026 | 5 |  |
2027 to 2031 | 22 |  |
Southern Power | Pension plans |  |  |
Components of net periodic |  |  |
Service cost | 10 | 8 | 7
Interest cost | 5 | 6 | 5
Expected return on plan assets | (14) | (13) | (10)
Recognized net loss | 3 | 2 | 1
Net amortization | 0 | 0 | 0
Prior service cost | 0 | 0 | 0
Total change | 4 | 3 | 3
Benefit payments: |  |  |
2022 | 6 |  |
2023 | 6 |  |
2024 | 6 |  |
2025 | 7 |  |
2026 | 7 |  |
2027 to 2031 | 42 |  |
Southern Power | Other postretirement benefit plans |  |  |
Components of net periodic |  |  |
Service cost | 0 | 1 | 1
Interest cost | 0 | 0 | 0
Expected return on plan assets | 1 | 0 | 0
Prior service cost | 0 | 0 | 0
Total change | 1 | 1 | 1
Benefit payments: |  |  |
2022 | 0 |  |
2023 | 0 |  |
2024 | 0 |  |
2025 | 1 |  |
2026 | 1 |  |
2027 to 2031 | 3 |  |
Southern Company Gas | Pension plans |  |  |
Components of net periodic |  |  |
Service cost | 37 | 33 | 25
Interest cost | 24 | 31 | 36
Expected return on plan assets | (86) | (75) | (60)
Recognized net loss | 13 | 6 | 2
Net amortization | 15 | 15 | 14
Prior service cost | (3) | (3) | (3)
Total change | 0 | 7 | 14
Benefit payments: |  |  |
2022 | 65 |  |
2023 | 65 |  |
2024 | 64 |  |
2025 | 64 |  |
2026 | 64 |  |
2027 to 2031 | 329 |  |
Southern Company Gas | Other postretirement benefit plans |  |  |
Components of net periodic |  |  |
Service cost | 2 | 2 | 1
Interest cost | 4 | 7 | 9
Expected return on plan assets | (10) | (10) | (7)
Prior service cost | 6 | 6 | 6
Total change | 2 | $ 5 | $ 9
Benefit payments: |  |  |
2022 | 17 |  |
2023 | 17 |  |
2024 | 18 |  |
2025 | 17 |  |
2026 | 17 |  |
2027 to 2031 | $ 75 |  |

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