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, 2020	Dec. 31, 2019	Dec. 31, 2018
Net sales:
Net sales	$ 1,172.7	$ 1,514.1	$ 1,585.9
Aero/HS products
Net sales:
Net sales	537.9	803.2	739.4
Automotive Extrusions
Net sales:
Net sales	161.4	190.5	239.3
GE products
Net sales:
Net sales	458.8	480.1	546.0
Other products
Net sales:
Net sales	14.6	40.3	61.2
Transferred at Point in Time
Net sales:
Net sales	690.9	866.9	912.7
Transferred over Time
Net sales:
Net sales	$ 481.8	$ 647.2	$ 673.2