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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, 2020 | Dec. 31, 2019
Pension Plan [Member] |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | $ 749.0 | $ 687.0
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 5,332.0 | 749.0
Pension Plan [Member] | Private Equity Funds [Member] |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 50.0 |
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 114.0 | 50.0
Reserve for investment commitments | 86.0 |
Other Postretirement Benefits Plan [Member] |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 260.0 | 240.0
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 783.0 | 260.0
Other Postretirement Benefits Plan [Member] | Private Equity Funds [Member] |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 2.0 |
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 3.0 | 2.0
Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member] |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 212.0 | 229.0
Actual return on plan assets: |  |
Relating to assets still held at the reporting date | 8.0 | (1.0)
Relating to assets sold during the period | 6.0 | 30.0
Purchases | 195.0 | 17.0
Sales | (13.0) | (60.0)
Acquired through business combinations | 262.0 |
Other | 0.0 | (3.0)
Fair value of plan assets — end of year | 670.0 | 212.0
Fair Value, Inputs, Level 3 [Member] | Pension Plan [Member] | Private Equity Funds [Member] |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 50.0 |
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 114.0 | 50.0
Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member] |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 34.0 |
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 38.0 | 34.0
Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member] | Private Equity Funds [Member] |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 2.0 |
Actual return on plan assets: |  |
Fair value of plan assets — end of year | 3.0 | 2.0
Fair Value, Inputs, Level 3 [Member] | Other Postretirement Benefits Plan [Member] | Veba Trust [Member] |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] |  |
Fair value of plan assets — beginning of year | 34.0 | 36.0
Actual return on plan assets: |  |
Relating to assets still held at the reporting date | 2.0 | 1.0
Relating to assets sold during the period | 1.0 | 0.0
Purchases | 0.0 | 0.0
Sales | (1.0) | (3.0)
Acquired through business combinations | 2.0 |
Other | 0.0 | 0.0
Fair value of plan assets — end of year | $ 38.0 | $ 34.0

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