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Employee Benefits - Benefit Obligations and Funded Status of Pension Plans, OPEB and Salaried VEBAs (Parenthetical) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2023
Defined Benefit Plan Disclosure [Line Items] |  |  |
Actuarial gain (loss) due to change in discount rate |  | $ 2.2 | $ (0.5)
Actuarial gain (loss) due to changes in census data |  | (0.3) | 0.3
Actuarial gain (loss) recognized in conjuction |  | (0.3) |
Pension and Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan | $ 9.8 | 9.8 | 11.3
Other Assets |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan | 1.6 | 1.6 | 1.3
Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Actuarial gain (loss) due to change in discount rate |  | 3.3 | (1.4)
Actuarial gain (loss) due to changes in census data |  | 6.5 | 2.5
Actuarial gain (loss) due to changes in projected depletion year |  | (2.9) | 2.7
Actuarial gain (loss) due to changes in morbidity assumptions |  | 3.2 |
Actuarial gain (loss) due to change in trend rate |  | (3.3) | (3.1)
Defined Benefit Plan, Funded (Unfunded) Status of Plan | (64.7) | (64.7) | (68.8)
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 | 3.1 | 3.1 | 3.3
Other Postretirement Benefits Plan | Pension and Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan | 61.6 | 61.6 | 65.5
Pension Plans |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Prior service cost (credit) |  | 2.2 | 6.6
Defined Benefit Plan, Funded (Unfunded) Status of Plan | (8.2) | (8.2) | (10.0)
Salaried VEBA |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Prior service cost (credit) |  |  | (8.8)
Actuarial gain (loss) due to change in discount rate |  | 1.4 | (0.7)
Actuarial gain (loss) due to changes in census data |  | 0.1 | 0.3
Actuarial gain (loss) due to change in trend rate |  | $ (0.1) |
Defined Benefit Plan, Funded (Unfunded) Status of Plan |  |  | $ (3.8)
Kaiser Aluminum Canada Limited Retirement Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plan, obligation associated for salaried employees | $ 4.5 |  |

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