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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net sales:
Net sales	$ 255.7	$ 374.9	$ 900.7	$ 1,145.4
Transferred at Point in Time
Net sales:
Net sales	151.8	212.5	532.1	658.4
Transferred over Time
Net sales:
Net sales	103.9	162.4	368.6	487.0
Aero/HS products
Net sales:
Net sales	100.9	198.9	441.5	586.8
Automotive Extrusions
Net sales:
Net sales	46.2	47.5	110.3	146.0
GE products
Net sales:
Net sales	105.1	117.1	337.6	375.3
Other products
Net sales:
Net sales	$ 3.5	$ 11.4	$ 11.3	$ 37.3