Other Income, Net - Components of Other Income, Net (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Other Income and Expenses [Abstract]
Interest income		$ 3.6	$ 1.7	$ 1.3
Net periodic postretirement and postemployment benefit cost		(4.3)	(8.4)	(5.6)
Unrealized gain (loss) on equity securities		0.3	0.6	(1.2)
Gain on disposition of property, plant and equipment		3.8	13.8	6.0
Gain on business interruption insurance recoveries	[1]	$ 16.3	$ 0.0	$ 0.0
Gain on Business Interruption Insurance Recovery, Statement of Income or Comprehensive Income [Extensible Enumeration]		Costs and Expenses	Costs and Expenses	Costs and Expenses
Post-acquisition funding received from Alcoa Corporation	[2]	$ 0.0	$ 0.0	$ 6.0
All other, net		(0.2)	(0.3)	(0.1)
Other income, net		$ 19.5	$ 7.4	$ 6.4

[1] Represents advances against business interruption insurance claims. We recognize such advances in the period in which the insurance proceeds are received or become realizable. As of December 31, 2024 we received net cash proceeds of $15.9 million.

[2] Represents reimbursement received for repairs and maintenance expenditures on certain machinery and equipment that we had purchased from Alcoa in connection with our March 31, 2021 acquisition of Warrick.