Business, Product and Geographical Area Information - Net Sales by End Market Segment Applications (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Net sales:
Net sales	$ 773.4	$ 814.1	$ 1,510.9	$ 1,621.7
Aero/HS Products
Net sales:
Net sales	226.1	225.1	446.6	439.1
Packaging
Net sales:
Net sales	312.4	354.7	610.5	708.9
GE Products
Net sales:
Net sales	162.6	159.4	315.6	321.5
Automotive Extrusions
Net sales:
Net sales	69.7	68.4	133.2	139.2
Other products
Net sales:
Net sales	2.6	6.5	5.0	13.0
Products Transferred at a Point in Time
Net sales:
Net sales	591.8	622.6	1,161.2	1,247.4
Products Transferred Over Time
Net sales:
Net sales	$ 181.6	$ 191.5	$ 349.7	$ 374.3