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RETIREMENT BENEFITS - Changes in Projected Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Pension plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | $ 12,602,000,000 | $ 16,382,000,000 |
Service cost | 275,000,000 | 412,000,000 | $ 434,000,000
Interest cost | 626,000,000 | 408,000,000 | 346,000,000
Benefits paid | (744,000,000) | (692,000,000) |
Actuarial loss | 493,000,000 | (3,908,000,000) |
Balance at end of year | 13,252,000,000 | 12,602,000,000 | 16,382,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 14,218,000,000 | 17,225,000,000 |
Actual loss on plan assets | 1,092,000,000 | (2,376,000,000) |
Employer contributions | 52,000,000 | 61,000,000 |
Benefits paid | (744,000,000) | (692,000,000) |
Fair value of plan assets at end of year | 14,618,000,000 | 14,218,000,000 | 17,225,000,000
Accrued asset | 1,366,000,000 | 1,616,000,000 |
Other postretirement benefit plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | 1,441,000,000 | 1,849,000,000 |
Service cost | 15,000,000 | 23,000,000 | 24,000,000
Interest cost | 70,000,000 | 42,000,000 | 35,000,000
Benefits paid | (107,000,000) | (109,000,000) |
Actuarial loss | (33,000,000) | (365,000,000) |
Retiree drug subsidy |  | 1,000,000 |
Balance at end of year | 1,386,000,000 | 1,441,000,000 | 1,849,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 998,000,000 | 1,251,000,000 |
Actual loss on plan assets | 131,000,000 | (218,000,000) |
Employer contributions | 73,000,000 | 73,000,000 |
Benefits paid | (107,000,000) | (108,000,000) |
Fair value of plan assets at end of year | 1,095,000,000 | 998,000,000 | 1,251,000,000
Accrued asset | (291,000,000) | (443,000,000) |
Alabama Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | 2,906,000,000 | 3,806,000,000 |
Service cost | 64,000,000 | 99,000,000 | 102,000,000
Interest cost | 145,000,000 | 96,000,000 | 82,000,000
Benefits paid | (155,000,000) | (144,000,000) |
Actuarial loss | 116,000,000 | (951,000,000) |
Balance at end of year | 3,076,000,000 | 2,906,000,000 | 3,806,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 3,427,000,000 | 4,141,000,000 |
Actual loss on plan assets | 260,000,000 | (579,000,000) |
Employer contributions | 11,000,000 | 9,000,000 |
Benefits paid | (154,000,000) | (144,000,000) |
Fair value of plan assets at end of year | 3,544,000,000 | 3,427,000,000 | 4,141,000,000
Accrued asset | 468,000,000 | 521,000,000 |
Alabama Power | Other postretirement benefit plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | 344,000,000 | 440,000,000 |
Service cost | 4,000,000 | 6,000,000 | 6,000,000
Interest cost | 17,000,000 | 10,000,000 | 9,000,000
Benefits paid | (24,000,000) | (23,000,000) |
Actuarial loss | (12,000,000) | (89,000,000) |
Retiree drug subsidy |  | 0 |
Balance at end of year | 329,000,000 | 344,000,000 | 440,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 372,000,000 | 489,000,000 |
Actual loss on plan assets | 52,000,000 | (98,000,000) |
Employer contributions | 3,000,000 | 4,000,000 |
Benefits paid | (24,000,000) | (23,000,000) |
Fair value of plan assets at end of year | 403,000,000 | 372,000,000 | 489,000,000
Accrued asset | 74,000,000 | 28,000,000 |
Georgia Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | 3,851,000,000 | 5,012,000,000 |
Service cost | 68,000,000 | 103,000,000 | 112,000,000
Interest cost | 191,000,000 | 123,000,000 | 104,000,000
Benefits paid | (224,000,000) | (226,000,000) |
Actuarial loss | 123,000,000 | (1,161,000,000) |
Balance at end of year | 4,009,000,000 | 3,851,000,000 | 5,012,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 4,456,000,000 | 5,415,000,000 |
Actual loss on plan assets | 331,000,000 | (753,000,000) |
Employer contributions | 9,000,000 | 20,000,000 |
Benefits paid | (225,000,000) | (226,000,000) |
Fair value of plan assets at end of year | 4,571,000,000 | 4,456,000,000 | 5,415,000,000
Accrued asset | 562,000,000 | 605,000,000 |
Georgia Power | Other postretirement benefit plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | 514,000,000 | 656,000,000 |
Service cost | 4,000,000 | 6,000,000 | 7,000,000
Interest cost | 25,000,000 | 15,000,000 | 12,000,000
Benefits paid | (36,000,000) | (38,000,000) |
Actuarial loss | (18,000,000) | (125,000,000) |
Retiree drug subsidy |  | 0 |
Balance at end of year | 489,000,000 | 514,000,000 | 656,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 368,000,000 | 450,000,000 |
Actual loss on plan assets | 51,000,000 | (71,000,000) |
Employer contributions | 27,000,000 | 27,000,000 |
Benefits paid | (36,000,000) | (38,000,000) |
Fair value of plan assets at end of year | 410,000,000 | 368,000,000 | 450,000,000
Accrued asset | (79,000,000) | (146,000,000) |
Mississippi Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | 569,000,000 | 743,000,000 |
Service cost | 11,000,000 | 17,000,000 | 18,000,000
Interest cost | 28,000,000 | 18,000,000 | 16,000,000
Benefits paid | (32,000,000) | (30,000,000) |
Actuarial loss | 23,000,000 | (179,000,000) |
Balance at end of year | 599,000,000 | 569,000,000 | 743,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 649,000,000 | 786,000,000 |
Actual loss on plan assets | 50,000,000 | (110,000,000) |
Employer contributions | 2,000,000 | 3,000,000 |
Benefits paid | (32,000,000) | (30,000,000) |
Fair value of plan assets at end of year | 669,000,000 | 649,000,000 | 786,000,000
Accrued asset | 70,000,000 | 80,000,000 |
Mississippi Power | Other postretirement benefit plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | 59,000,000 | 76,000,000 |
Service cost | 1,000,000 | 1,000,000 | 1,000,000
Interest cost | 3,000,000 | 2,000,000 | 1,000,000
Benefits paid | (4,000,000) | (4,000,000) |
Actuarial loss | (2,000,000) | (16,000,000) |
Retiree drug subsidy |  | 0 |
Balance at end of year | 57,000,000 | 59,000,000 | 76,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 24,000,000 | 29,000,000 |
Actual loss on plan assets | 2,000,000 | (4,000,000) |
Employer contributions | 3,000,000 | 3,000,000 |
Benefits paid | (4,000,000) | (4,000,000) |
Fair value of plan assets at end of year | 25,000,000 | 24,000,000 | 29,000,000
Accrued asset | (32,000,000) | (35,000,000) |
Southern Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | 163,000,000 | 222,000,000 |
Service cost | 6,000,000 | 9,000,000 | 10,000,000
Interest cost | 8,000,000 | 6,000,000 | 5,000,000
Benefits paid | (6,000,000) | (5,000,000) |
Actuarial loss | 6,000,000 | (69,000,000) |
Balance at end of year | 177,000,000 | 163,000,000 | 222,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 178,000,000 | 213,000,000 |
Actual loss on plan assets | 12,000,000 | (31,000,000) |
Employer contributions | 2,000,000 | 1,000,000 |
Benefits paid | (7,000,000) | (5,000,000) |
Fair value of plan assets at end of year | 185,000,000 | 178,000,000 | 213,000,000
Accrued asset | 8,000,000 | 15,000,000 |
Southern Power | Other postretirement benefit plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | 9,000,000 | 11,000,000 |
Service cost | 0 | 0 | 0
Interest cost | 0 | 0 | 0
Benefits paid | (1,000,000) | (1,000,000) |
Actuarial loss | 1,000,000 | (1,000,000) |
Retiree drug subsidy |  | 0 |
Balance at end of year | 9,000,000 | 9,000,000 | 11,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 0 | 0 |
Actual loss on plan assets | 0 | 0 |
Employer contributions | 1,000,000 | 1,000,000 |
Benefits paid | (1,000,000) | (1,000,000) |
Fair value of plan assets at end of year | 0 | 0 | 0
Accrued asset | (9,000,000) | (9,000,000) |
Southern Company Gas | Pension plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | 868,000,000 | 1,134,000,000 |
Service cost | 24,000,000 | 34,000,000 | 37,000,000
Interest cost | 42,000,000 | 28,000,000 | 24,000,000
Benefits paid | (104,000,000) | (75,000,000) |
Actuarial loss | 52,000,000 | (253,000,000) |
Balance at end of year | 882,000,000 | 868,000,000 | 1,134,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 1,002,000,000 | 1,241,000,000 |
Actual loss on plan assets | 79,000,000 | (167,000,000) |
Employer contributions | 3,000,000 | 3,000,000 |
Benefits paid | (104,000,000) | (75,000,000) |
Fair value of plan assets at end of year | 980,000,000 | 1,002,000,000 | 1,241,000,000
Accrued asset | 98,000,000 | 134,000,000 |
Southern Company Gas | Other postretirement benefit plans |  |  |
Change in benefit obligation |  |  |
Benefit obligation at beginning of year | 179,000,000 | 237,000,000 |
Service cost | 1,000,000 | 1,000,000 | 2,000,000
Interest cost | 9,000,000 | 5,000,000 | 4,000,000
Benefits paid | (18,000,000) | (18,000,000) |
Actuarial loss | 1,000,000 | (46,000,000) |
Retiree drug subsidy |  | 0 |
Balance at end of year | 172,000,000 | 179,000,000 | 237,000,000
Change in plan assets |  |  |
Fair value of plan assets at beginning of year | 113,000,000 | 143,000,000 |
Actual loss on plan assets | 19,000,000 | (25,000,000) |
Employer contributions | 14,000,000 | 13,000,000 |
Benefits paid | (18,000,000) | (18,000,000) |
Fair value of plan assets at end of year | 128,000,000 | 113,000,000 | $ 143,000,000
Accrued asset | (44,000,000) | $ (66,000,000) |
Qualified pension plan | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | 12,540,000,000 |  |
Change in plan assets |  |  |
Employer contributions | 0 |  |
Qualified pension plan | Alabama Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | 2,959,000,000 |  |
Qualified pension plan | Georgia Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | 3,865,000,000 |  |
Qualified pension plan | Mississippi Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | 569,000,000 |  |
Qualified pension plan | Southern Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | 154,000,000 |  |
Qualified pension plan | Southern Company Gas | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | 822,000,000 |  |
Non-qualified pension plan | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | 713,000,000 |  |
Non-qualified pension plan | Alabama Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | 117,000,000 |  |
Non-qualified pension plan | Georgia Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | 145,000,000 |  |
Non-qualified pension plan | Mississippi Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | 29,000,000 |  |
Non-qualified pension plan | Southern Power | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | 22,000,000 |  |
Non-qualified pension plan | Southern Company Gas | Pension plans |  |  |
Change in benefit obligation |  |  |
Balance at end of year | $ 60,000,000 |  |

X
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