Other Income (Expense), Net - Other Income (Expense), Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Other Income and Expenses [Abstract]
Interest income	$ 1.7	$ 1.3	$ 0.2
Net periodic postretirement benefit cost	(8.4)	(5.6)	(4.1)
Unrealized gain (loss) on equity securities	0.6	(1.2)	(0.2)
Loss on extinguishment of debt			(35.9)
Gain on disposition of property, plant and equipment	13.8	6.0	0.5
Post-acquisition funding received from Alcoa Corporation		6.0
All other, net	(0.3)	(0.1)	0.6
Other income (expense), net	$ 7.4	$ 6.4	$ (38.9)