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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net sales:
Net sales	$ 275.7	$ 375.3	$ 645.0	$ 770.5
Transferred at Point in Time
Net sales:
Net sales	164.7	222.6	380.3	445.9
Transferred over Time
Net sales:
Net sales	111.0	152.7	264.7	324.6
Aero/HS products
Net sales:
Net sales	144.6	189.4	340.6	387.9
Automotive Extrusions
Net sales:
Net sales	16.7	45.5	64.1	98.5
GE products
Net sales:
Net sales	110.8	128.2	232.5	258.2
Other products
Net sales:
Net sales	$ 3.6	$ 12.2	$ 7.8	$ 25.9