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Employee Benefits - Benefit Obligations and Funded Status of Pension Plans, OPEB and Salaried VEBAs (Parenthetical) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items] |  |
Actuarial gain (loss) due to change in discount rate | $ (0.5) | $ 8.0
Actuarial gain (loss) due to changes in census data | 0.3 | (0.9)
Pension and Other Postretirement Benefits |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan | 11.3 | 4.9
Other Assets |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan | 1.3 | 1.0
Other Postretirement Benefits Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actuarial gain (loss) due to change in discount rate | (1.4) | 20.4
Actuarial gain (loss) due to changes in census data | 2.5 | 0.7
Actuarial gain (loss) due to changes in projected depletion year | 2.7 |
Actuarial gain (loss) due to change in trend rate | (3.1) | (0.7)
Defined Benefit Plan, Funded (Unfunded) Status of Plan | (68.8) | (66.4)
Other Postretirement Benefits Plan | Accrued Salaries and Wages and Related Expenses |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan | 3.3 | 2.0
Other Postretirement Benefits Plan | Pension and Other Postretirement Benefits |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan | 65.5 | 64.4
Pension Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Prior service cost (credit) | 6.6 |
Defined Benefit Plan, Funded (Unfunded) Status of Plan | (10.0) | (3.9)
Salaried VEBA |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Prior service cost (credit) | (8.8) |
Actuarial gain (loss) due to change in discount rate | (0.7) | 12.6
Actuarial gain (loss) due to changes in census data | 0.3 | 2.6
Actuarial gain (loss) due to change in trend rate |  | 1.1
Defined Benefit Plan, Funded (Unfunded) Status of Plan | $ (3.8) | $ (16.5)

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