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RETIREMENT BENEFITS - Changes in Regulatory Assets (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 | $ 5,833 |  |
Total other comprehensive income (loss) | (10) | $ 71 | $ 158
Ending balance | 6,523 | 5,833 |
Regulatory asset | 66 | 216 |
Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Total other comprehensive income (loss) | (38) |  |
Pension plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 2,371 |  |
Ending balance | 2,913 | 2,371 |
Pension plans | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 2,371 | 2,801 |
Reclassification adjustments | (7) |  |
Total other comprehensive income (loss) | 55 | (76) |
Ending balance | 2,913 | 2,371 | 2,801
Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (34) | (247) |
Total other comprehensive income (loss) | 542 | (430) |
Pension plans | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 62 | (82) |
Reclassification adjustments | (8) | 6 |
Pension plans | Net gain | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 576 | (183) |
Reclassification adjustments | (33) | (246) |
Pension plans | Change in prior service costs |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 1 |  |
Pension plans | Change in prior service costs | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (1) | (1) |
Pension plans | Amortization of regulatory assets | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Other Postretirement Benefits Plan |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 136 |  |
Reclassification adjustments | 7 | (1) |
Ending balance | 203 | 136 |
Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | (136) | (74) |
Total other comprehensive income (loss) | (5) | (4) |
Ending balance | (208) | (136) | (74)
Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 5 | 2 |
Total other comprehensive income (loss) | (72) | (62) |
Other Postretirement Benefits Plan | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | (12) | (3) |
Other Postretirement Benefits Plan | Net gain | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 77 | 64 |
Reclassification adjustments | 6 | 1 |
Other Postretirement Benefits Plan | Change in prior service costs | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (1) | 1 |
Other Postretirement Benefits Plan | Amortization of regulatory assets | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Alabama Power |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 2,340 |  |
Total other comprehensive income (loss) | 2 | 4 | 6
Ending balance | 2,720 | 2,340 |
Regulatory asset | 76 | 97 |
Alabama Power | Pension plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 679 |  |
Ending balance | 821 | 679 |
Alabama Power | Pension plans | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 679 | 809 |
Ending balance | 821 | 679 | 809
Alabama Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (11) | (63) |
Total other comprehensive income (loss) | 142 | (130) |
Alabama Power | Pension plans | Net gain | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 153 | (67) |
Reclassification adjustments | (10) | (62) |
Alabama Power | Pension plans | Change in prior service costs | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (1) | (1) |
Alabama Power | Pension plans | Amortization of regulatory assets | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Alabama Power | Other Postretirement Benefits Plan |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 21 |  |
Ending balance | 48 | 21 |
Alabama 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 | (21) | (62) |
Ending balance | (48) | (21) | (62)
Alabama Power | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 3 | 0 |
Total other comprehensive income (loss) | (27) | 41 |
Alabama Power | Other Postretirement Benefits Plan | Net gain | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 30 | (41) |
Reclassification adjustments | 3 | 0 |
Alabama Power | Other Postretirement Benefits Plan | Change in prior service costs | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Alabama Power | Other Postretirement Benefits Plan | Amortization of regulatory assets | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Georgia Power |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 4,663 |  |
Total other comprehensive income (loss) | 3 | 28 | 6
Ending balance | 5,090 | 4,663 |
Regulatory asset |  | 83 |
Georgia Power | Pension plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 887 |  |
Ending balance | 1,051 | 887 |
Georgia Power | Pension plans | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 887 | 971 |
Ending balance | 1,051 | 887 | 971
Georgia Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (14) | (75) |
Total other comprehensive income (loss) | 164 | (84) |
Georgia Power | Pension plans | Net gain | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 178 | (9) |
Reclassification adjustments | (13) | (74) |
Georgia Power | Pension plans | Change in prior service costs | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (1) | (1) |
Georgia Power | Pension plans | Amortization of regulatory assets | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Georgia Power | Other Postretirement Benefits Plan |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 39 |  |
Ending balance | 74 | 39 |
Georgia 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 | (39) | (10) |
Ending balance | (74) | (39) | (10)
Georgia Power | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 3 | (2) |
Total other comprehensive income (loss) | (35) | (29) |
Georgia Power | Other Postretirement Benefits Plan | Net gain | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 38 | 27 |
Reclassification adjustments | 4 | (2) |
Georgia Power | Other Postretirement Benefits Plan | Change in prior service costs | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (1) | 0 |
Georgia Power | Other Postretirement Benefits Plan | Amortization of regulatory assets | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Mississippi Power |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 4 |  |
Total other comprehensive income (loss) | 0 | 0 | 1
Ending balance | 90 | 4 |
Regulatory asset | 44 | 36 |
Mississippi Power | Pension plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 123 |  |
Ending balance | 152 | 123 |
Mississippi Power | Pension plans | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 123 | 146 |
Ending balance | 152 | 123 | 146
Mississippi Power | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (2) | (11) |
Total other comprehensive income (loss) | 29 | (23) |
Mississippi Power | Pension plans | Net gain | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 31 | (12) |
Reclassification adjustments | (2) | (11) |
Mississippi Power | Pension plans | Change in prior service costs | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Mississippi Power | Pension plans | Amortization of regulatory assets | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Mississippi Power | Other Postretirement Benefits Plan |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | (9) |  |
Ending balance | 10 | (9) |
Mississippi 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 | (9) | 1 |
Ending balance | (10) | (9) | 1
Mississippi Power | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Total other comprehensive income (loss) | (1) | (10) |
Mississippi Power | Other Postretirement Benefits Plan | Net gain | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 1 | 10 |
Reclassification adjustments | 0 | 0 |
Mississippi Power | Other Postretirement Benefits Plan | Change in prior service costs | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Mississippi Power | Other Postretirement Benefits Plan | Amortization of regulatory assets | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Southern Power |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Total other comprehensive income (loss) | 1 | 10 | 40
Southern Power | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Total other comprehensive income (loss) | (7) |  |
Southern Power | Pension plans | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 |  |
Total other comprehensive income (loss) | 9 | (24) |
Southern Power | Pension plans | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 9 | (22) |
Reclassification adjustments | 0 | (2) |
Southern Power | Pension plans | Change in prior service costs |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 |  |
Southern Power | Other Postretirement Benefits Plan |  |  |
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] |  |  |
Total other comprehensive income (loss) | 1 | (2) |
Southern Power | Other Postretirement Benefits Plan | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 1 | (2) |
Southern Company Gas |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | (1,891) |  |
Total other comprehensive income (loss) | (15) | 7 | 46
Ending balance | (2,225) | (1,891) |
Southern Company Gas | Pension and Other Postretirement Benefit Plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Total other comprehensive income (loss) | (16) |  |
Southern Company Gas | Pension plans |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 111 |  |
Ending balance | 143 | 111 |
Regulatory asset | 173 | 190 |
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 | 111 | 91 |
Reclassification adjustments | 1 |  |
Total other comprehensive income (loss) | 30 | (30) |
Ending balance | 143 | 111 | 91
Southern Company Gas | Pension plans | Pension and Other Postretirement Benefit Plans | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (2) | (7) |
Total other comprehensive income (loss) | 32 | 20 |
Southern Company Gas | Pension plans | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 29 | (30) |
Reclassification adjustments | 0 | 0 |
Southern Company Gas | Pension plans | Net gain | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 34 | 27 |
Reclassification adjustments | 0 | 1 |
Southern Company Gas | Pension plans | Change in prior service costs |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 1 |  |
Southern Company Gas | Pension plans | Change in prior service costs | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 2 | 2 |
Southern Company Gas | Pension plans | Amortization of regulatory assets | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (4) | (10) |
Southern Company Gas | Other Postretirement Benefits Plan |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Beginning balance | 58 |  |
Reclassification adjustments | (8) | 3 |
Ending balance | 68 | 58 |
Regulatory asset | 24 | 32 |
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 | (58) | (34) |
Total other comprehensive income (loss) | (8) | 3 |
Ending balance | (68) | (58) | $ (34)
Southern Company Gas | Other Postretirement Benefits Plan | Pension and Other Postretirement Benefit Plans | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | (10) | (11) |
Total other comprehensive income (loss) | (10) | (24) |
Southern Company Gas | Other Postretirement Benefits Plan | Net gain |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 0 | 0 |
Southern Company Gas | Other Postretirement Benefits Plan | Net gain | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Net gain (loss) | 0 | 13 |
Reclassification adjustments | 0 | 0 |
Southern Company Gas | Other Postretirement Benefits Plan | Change in prior service costs | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | 0 | 0 |
Southern Company Gas | Other Postretirement Benefits Plan | Amortization of regulatory assets | Regulatory Assets |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |
Reclassification adjustments | $ (10) | $ (11) |

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