Business, Product and Geographical Area Information - Schedule of Net Sales by End Market Segment Applications (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Net sales:
Net sales	$ 777.4	$ 737.5	[1]
Aero/HS Products
Net sales:
Net sales	214.7	220.5
Packaging
Net sales:
Net sales	314.2	298.1
GE Products
Net sales:
Net sales	181.6	153.0
Automotive Extrusions
Net sales:
Net sales	66.9	63.5
Other Products
Net sales:
Net sales	0.0	2.4
Products Transferred at a Point in Time
Net sales:
Net sales	590.8	569.4
Products Transferred Over Time
Net sales:
Net sales	$ 186.6	$ 168.1

[1] Adjusted to reflect the retrospective change in inventory valuation methodology from LIFO to WAC. See Note 14 for further discussion.