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RETIREMENT BENEFITS - Components of Accumulated OCI (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | $ 34,532 | $ 32,276 | $ 32,234
Total other comprehensive income (loss) | (10) | 71 | 158
Ending balance | 35,225 | 34,532 | 32,276
Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | (18) |  |
Total other comprehensive income (loss) | (38) |  |
Ending balance | (56) | (18) |
Pension plans |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Prior service cost | (1) | (2) |
Net loss | 80 | 26 |
AOCI | 79 | 24 |
Pension plans | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 24 | 100 |
Reclassification adjustments | (7) |  |
Total other comprehensive income (loss) | 55 | (76) |
Ending balance | 79 | 24 | 100
Pension plans | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain | 62 | (82) |
Reclassification adjustments | (8) | 6 |
Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Prior service cost | 1 | 1 |
Net loss | (10) | (5) |
AOCI | (9) | (4) |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 7 | (1) |
Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | (4) | 0 |
Total other comprehensive income (loss) | (5) | (4) |
Ending balance | (9) | (4) | 0
Other Postretirement Benefits Plan | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain | (12) | (3) |
Southern Power |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 6,916 | 6,598 | 6,631
Total other comprehensive income (loss) | 1 | 10 | 40
Ending balance | 6,698 | 6,916 | 6,598
Southern Power | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | (9) |  |
Total other comprehensive income (loss) | (7) |  |
Ending balance | (16) | (9) |
Southern Power | Pension plans |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Prior service cost | 0 | 0 |
Net loss | 20 | 11 |
AOCI | 20 | 11 |
Southern Power | Pension plans | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 11 | 35 |
Reclassification adjustments | 0 |  |
Total other comprehensive income (loss) | 9 | (24) |
Ending balance | 20 | 11 | 35
Southern Power | Pension plans | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain | 9 | (22) |
Reclassification adjustments | 0 | (2) |
Southern Power | Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Prior service cost | 0 | 0 |
Net loss | 1 | 0 |
AOCI | 1 | 0 |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Southern Power | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 0 | 2 |
Total other comprehensive income (loss) | 1 | (2) |
Ending balance | 1 | 0 | 2
Southern Power | Other Postretirement Benefits Plan | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain | 1 | (2) |
Southern Company Gas |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Total other comprehensive income (loss) | (15) | 7 | 46
Southern Company Gas | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 56 |  |
Total other comprehensive income (loss) | (16) |  |
Ending balance | 40 | 56 |
Southern Company Gas | Pension plans |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Prior service cost | (2) | (3) |
Net loss | (43) | (72) |
AOCI | (45) | (75) |
Southern Company Gas | Pension plans | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | (75) | (45) |
Reclassification adjustments | 1 |  |
Total other comprehensive income (loss) | 30 | (30) |
Ending balance | (45) | (75) | (45)
Southern Company Gas | Pension plans | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain | 29 | (30) |
Reclassification adjustments | 0 | 0 |
Southern Company Gas | Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Prior service cost | 0 | 0 |
Net loss | (10) | (2) |
AOCI | (10) | (2) |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (8) | 3 |
Southern Company Gas | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | (2) | (5) |
Total other comprehensive income (loss) | (8) | 3 |
Ending balance | (10) | (2) | $ (5)
Southern Company Gas | Other Postretirement Benefits Plan | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain | $ 0 | $ 0 |

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