Business, Product and Geographical Area Information - Net Sales by End Market Segment Applications (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Net sales:
Net sales	$ 737.5	$ 807.6
Aero/HS Products
Net sales:
Net sales	220.5	214.0
Packaging
Net sales:
Net sales	298.1	354.2
GE Products
Net sales:
Net sales	153.0	162.1
Automotive Extrusions
Net sales:
Net sales	63.5	70.8
Other products
Net sales:
Net sales	2.4	6.5
Products Transferred at a Point in Time
Net sales:
Net sales	569.4	624.8
Products Transferred Over Time
Net sales:
Net sales	$ 168.1	$ 182.8