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PENSION AND OTHER POST-EMPLOYMENT BENEFITS - Narrative (Details) - USD ($) $ in Millions |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Feb. 01, 2019 | Jan. 31, 2018 | Dec. 31, 2020 | Sep. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Mark-to-market adjustment, net of capitalized amounts |  |  | $ 54 |  | $ 423 |  | $ 477 | $ 676 | $ 145
Increase in discount rate |  |  | 0.29% |  |  |  | 0.38% |  |
Company contributions | $ 500 |  |  |  |  |  |  |  |
Future contributions made in current year |  | $ 750 |  |  |  |  |  |  |
Return seeking assets sold during period |  |  |  | $ 800 |  |  | $ 1,400 |  |
Transmission companies |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Mark-to-market adjustment, net of capitalized amounts |  |  | $ 21 |  |  |  |  |  |
Discontinued Operations |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Mark-to-market adjustment, net of capitalized amounts |  |  |  |  |  |  |  | 2 | 1
Pensions and OPEB |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Actual return on plan assets |  |  |  |  |  |  | $ 1,225 | $ 1,492 | $ 371
Actual return on plan assets (percent) |  |  |  |  |  |  | 14.70% | 20.20% | (4.00%)
Expected return on plan assets |  |  |  |  |  |  | $ 651 | $ 569 | $ 605
Pension |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Company contributions |  |  |  |  |  |  | 24 | 521 |
Actual return on plan assets |  |  |  |  |  |  | $ 1,165 | $ 1,419 |
Expected long-term return on plan assets |  |  |  |  |  |  | 7.50% | 7.50% | 7.50%
Expected return on plan assets |  |  |  |  |  |  | $ 618 | $ 540 | $ 574
Increase in benefit obligation due to RP2014 mortality table |  |  |  |  |  |  | 74 |  |
Pension | Forecast |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Expected long-term return on plan assets |  |  |  |  |  | 7.50% |  |  |
OPEB |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Company contributions |  |  |  |  |  |  | 23 | 21 |
Actual return on plan assets |  |  |  |  |  |  | $ 60 | $ 73 |
Expected long-term return on plan assets |  |  |  |  |  |  | 7.50% | 7.50% | 7.50%
Expected return on plan assets |  |  |  |  |  |  | $ 33 | $ 29 | $ 31
Increase in benefit obligation due to RP2014 mortality table |  |  |  |  |  |  | 2 |  |
OPEB | Transmission companies |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Mark-to-market adjustment, net of capitalized amounts |  |  |  |  |  |  | $ 40 | $ 47 | $ 8
Minimum |  |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |
Company contributions |  | $ 500 |  |  |  |  |  |  |

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