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EMPLOYEE BENEFITS - Schedule of Asset Category (Details) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | --- | ---
Pension Plan |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | $ 6,495.5 | $ 5,409.2 | $ 4,968.6
Uncategorized | 5,479.8 | 4,480.4 |
Pension Plan | Total Assets Before 401 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 6,794.0 | 5,671.1 |
Pension Plan | Equity securities |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 2,107.7 | 1,952.5 |
Uncategorized | 1,385.2 | 1,321.7 |
Pension Plan | Fixed Income |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 2,062.5 | 1,781.7 |
Uncategorized | 1,689.1 | 1,402.5 |
Pension Plan | Private Equity |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 1,702.7 | 1,197.7 |
Uncategorized | 1,702.7 | 1,175.4 |
Pension Plan | Real Assets |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 921.1 | 739.2 |
Uncategorized | 702.8 | 580.8 |
Pension Plan | 401 PBOP Assets |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | (298.5) | (261.9) |
Pension Plan | Level 1 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 1,080.4 | 925.1 |
Pension Plan | Level 1 | Equity securities |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 722.5 | 630.8 |
Pension Plan | Level 1 | Fixed Income |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 139.6 | 113.6 |
Pension Plan | Level 1 | Private Equity |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 0.0 | 22.3 |
Pension Plan | Level 1 | Real Assets |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 218.3 | 158.4 |
Pension Plan | Level 2 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 233.8 | 265.6 |
Pension Plan | Level 2 | Equity securities |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 0.0 | 0.0 |
Pension Plan | Level 2 | Fixed Income |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 233.8 | 265.6 |
Pension Plan | Level 2 | Private Equity |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 0.0 | 0.0 |
Pension Plan | Level 2 | Real Assets |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 0.0 | 0.0 |
Other Postretirement Benefits Plan |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 1,138.3 | 1,004.1 | $ 935.9
Uncategorized | 463.5 | 424.4 |
Other Postretirement Benefits Plan | Total Assets Before 401 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 839.8 | 742.2 |
Other Postretirement Benefits Plan | Equity securities |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 439.7 | 394.3 |
Uncategorized | 248.3 | 217.8 |
Other Postretirement Benefits Plan | Fixed Income |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 220.4 | 212.1 |
Uncategorized | 125.5 | 152.9 |
Other Postretirement Benefits Plan | Private Equity |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 58.7 | 31.5 |
Uncategorized | 58.7 | 31.5 |
Other Postretirement Benefits Plan | Real Assets |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 121.0 | 104.3 |
Uncategorized | 31.0 | 22.2 |
Other Postretirement Benefits Plan | 401 PBOP Assets |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 298.5 | 261.9 |
Other Postretirement Benefits Plan | Level 1 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 331.1 | 274.6 |
Other Postretirement Benefits Plan | Level 1 | Equity securities |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 191.4 | 176.5 |
Other Postretirement Benefits Plan | Level 1 | Fixed Income |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 49.7 | 16.0 |
Other Postretirement Benefits Plan | Level 1 | Private Equity |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 0.0 | 0.0 |
Other Postretirement Benefits Plan | Level 1 | Real Assets |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 90.0 | 82.1 |
Other Postretirement Benefits Plan | Level 2 |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 45.2 | 43.2 |
Other Postretirement Benefits Plan | Level 2 | Equity securities |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 0.0 | 0.0 |
Other Postretirement Benefits Plan | Level 2 | Fixed Income |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 45.2 | 43.2 |
Other Postretirement Benefits Plan | Level 2 | Private Equity |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | 0.0 | 0.0 |
Other Postretirement Benefits Plan | Level 2 | Real Assets |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Fair value of pension plan assets | $ 0.0 | $ 0.0 |

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