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