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PENSIONS AND POSTRETIREMENT BENEFITS - Fair Value Measurements Using Significant Unobservable Inputs (Level 3) on Changes in Plan Assets (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022
Pension Benefits |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | $ 4,338 | $ 5,606
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 4,282 | 4,338
Pension Benefits | Private equity |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 143 |
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 146 | 143
OPEB |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 728 | 812
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 739 | 728
OPEB | Private equity |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 0 |
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 0 | 0
Significant Unobservable Inputs (Level 3) | Pension Benefits |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 912 | 904
Actual return on plan assets: |  |
Relating to assets still held at the reporting date | (16) | (6)
Relating to assets sold during the period | 10 | 15
Purchases | 24 | 28
Sales | (160) | (29)
Fair value of plan assets — end of year | 770 | 912
Significant Unobservable Inputs (Level 3) | Pension Benefits | Private equity |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 143 |
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 146 | 143
Significant Unobservable Inputs (Level 3) | OPEB |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 35 |
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 27 | 35
Significant Unobservable Inputs (Level 3) | OPEB | Private equity |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 0 |
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 0 | 0
Significant Unobservable Inputs (Level 3) | OPEB | VEBA Assets |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 35 | 42
Actual return on plan assets: |  |
Relating to assets still held at the reporting date | 2 | 1
Relating to assets sold during the period | 1 | 1
Purchases | 0 | 0
Sales | (11) | (9)
Fair value of plan assets — end of year | $ 27 | $ 35

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