Business, Product and Geographical Area Information and Concentration of Risk - Net Sales by End Market Segment Applications (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Net sales:
Net sales	$ 3,087.0	$ 3,427.9	$ 2,622.0
Aero/HS products
Net sales:
Net sales	899.3	676.1	533.7
Packaging
Net sales:
Net sales	1,315.2	1,585.3	1,119.3
Automotive Extrusions
Net sales:
Net sales	254.9	254.8	225.0
GE products
Net sales:
Net sales	596.5	883.8	706.1
Other products
Net sales:
Net sales	21.1	27.9	37.9
Transferred at Point in Time
Net sales:
Net sales	2,394.8	2,782.9	2,067.9
Transferred over Time
Net sales:
Net sales	$ 692.2	$ 645.0	$ 554.1