Business, Product and Geographical Area Information - Net Sales by End Market Segment Applications (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Net sales:
Net sales	$ 747.7	$ 743.6	$ 2,258.6	$ 2,365.3
Aero/HS Products
Net sales:
Net sales	213.1	223.3	659.7	662.4
Packaging
Net sales:
Net sales	319.5	312.2	930.0	1,021.1
GE Products
Net sales:
Net sales	150.7	143.6	466.3	465.1
Automotive Extrusions
Net sales:
Net sales	62.1	60.1	195.3	199.3
Other products
Net sales:
Net sales	2.3	4.4	7.3	17.4
Products Transferred at a Point in Time
Net sales:
Net sales	580.2	595.7	1,741.4	1,843.1
Products Transferred Over Time
Net sales:
Net sales	$ 167.5	$ 147.9	$ 517.2	$ 522.2