Business, Product and Geographical Area Information - Net Sales by End Market Segment Applications (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net sales:
Net sales	$ 324.0	$ 369.3
Transferred at Point in Time
Net sales:
Net sales	200.2	215.6
Transferred over Time
Net sales:
Net sales	123.8	153.7
Aero/HS products
Net sales:
Net sales	111.7	196.0
Automotive Extrusions
Net sales:
Net sales	57.6	47.4
GE products
Net sales:
Net sales	150.4	121.7
Other products
Net sales:
Net sales	$ 4.3	$ 4.2