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PENSION AND OTHER POST-EMPLOYMENT BENEFITS - Narrative (Details) $ in Millions |  | 1 Months Ended | 3 Months Ended | 8 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
May 12, 2023 USD ($) | Dec. 31, 2023 USD ($) numberOfEmployee | Dec. 31, 2023 USD ($) numberOfEmployee | Jun. 30, 2023 USD ($) | Dec. 31, 2023 USD ($) numberOfEmployee | Dec. 31, 2023 USD ($) numberOfEmployee | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | May 09, 2023 employee
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Percent of eligible employees that accepted the voluntary retirement program |  |  |  |  |  |  |  |  | 0.65
Number of employees | employee |  |  |  |  |  |  |  |  | 450
Number of separated employees | employee |  |  |  |  |  |  |  |  | 90
Defined benefit plan, assumptions used calculating benefit obligation, increase (decrease) in discount rate |  |  | 0.11% | 0.29% |  |  |  |  |
Pension |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Voluntary cash contribution | $ 750 |  |  |  |  |  |  |  |
Weighted-average discount rate |  |  |  |  | 8.00% |  |  |  |
Pension & OPEB mark-to-market |  |  | $ 137 | $ 59 |  | $ 108 | $ (98) | $ (253) |
Expected future contributions |  | $ 260 | $ 260 |  | $ 260 | $ 260 |  |  |
Defined benefit plan, number of covered employees by plan | numberOfEmployee |  | 1,900 | 1,900 |  | 1,900 | 1,900 |  |  |
Defined benefit plan, plan assets pre tax gain |  | $ 36 |  |  |  |  |  |  |
Pension | Banner Life Insurance Company and Reinsurance Group of America |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Defined benefit plan, plan assets, increase (decrease) for assets transferred to (from) plan |  | 683 |  |  |  |  |  |  |
Plan amendments |  | $ 719 |  |  |  |  |  |  |
OPEB |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Pension & OPEB mark-to-market |  |  |  |  |  | $ (30) | 26 | (129) |
OPEB | Regulated Transmission |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Mark-to-market adjustment, net of capitalized amounts |  |  |  |  |  | $ 36 | $ 15 | $ (31) |

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