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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Net sales:
Net sales	$ 741.0	$ 275.7	$ 1,065.0	$ 645.0
Transferred at Point in Time
Net sales:
Net sales	605.7	164.7	805.9	380.3
Transferred over Time
Net sales:
Net sales	135.3	111.0	259.1	264.7
Aero/HS products
Net sales:
Net sales	133.9	144.6	245.6	340.6
Packaging
Net sales:
Net sales	358.8		358.8
Automotive Extrusions
Net sales:
Net sales	55.6	16.7	113.2	64.1
GE products
Net sales:
Net sales	180.9	110.8	331.3	232.5
Other products
Net sales:
Net sales	$ 11.8	$ 3.6	$ 16.1	$ 7.8