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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net sales:
Net sales	$ 750.6	$ 255.7	$ 1,815.6	$ 900.7
Transferred at Point in Time
Net sales:
Net sales	613.6	151.8	1,419.5	532.1
Transferred over Time
Net sales:
Net sales	137.0	103.9	396.1	368.6
Aero/HS products
Net sales:
Net sales	142.0	100.9	387.6	441.5
Packaging
Net sales:
Net sales	367.3		726.1
Automotive Extrusions
Net sales:
Net sales	50.1	46.2	163.3	110.3
GE products
Net sales:
Net sales	187.1	105.1	518.4	337.6
Other products
Net sales:
Net sales	$ 4.1	$ 3.5	$ 20.2	$ 11.3