Other (Expense) Income, Net (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Other Income and Expenses [Abstract]
Interest income	$ 0.6	$ 0.4	$ 1.2	$ 0.6
Net periodic postretirement benefit cost	(2.2)	(1.4)	(6.1)	(4.3)
Unrealized (loss) gain on equity securities	(0.1)	(0.3)	0.2	(1.4)
(Loss) gain on disposition of property, plant and equipment	(0.2)	8.4	14.0	6.3
Post-acquisition funding received from Alcoa Corporation		6.0		6.0
All other, net	(0.3)	(0.4)	(0.4)	0.2
Other (expense) income, net	$ (2.2)	$ 12.7	$ 8.9	$ 7.4