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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net sales:
Net sales	$ 743.6	$ 748.9	$ 2,365.3	$ 2,651.9
Aero/HS products
Net sales:
Net sales	223.3	140.8	662.4	492.8
Packaging
Net sales:
Net sales	312.2	341.5	1,021.1	1,240.2
Automotive Extrusions
Net sales:
Net sales	60.1	60.6	199.3	196.0
GE products
Net sales:
Net sales	143.6	200.3	465.1	699.2
Other products
Net sales:
Net sales	4.4	5.7	17.4	23.7
Transferred at Point in Time
Net sales:
Net sales	595.7	630.2	1,843.1	2,165.1
Transferred over Time
Net sales:
Net sales	$ 147.9	$ 118.7	$ 522.2	$ 486.8