Business, Product and Geographical Area Information, Net Sales by End Market Segment Applications (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net Sales:
Net sales	$ 369.3	$ 395.2
Transferred at Point in Time
Net Sales:
Net sales	215.6	223.3
Transferred over Time
Net Sales:
Net sales	153.7	171.9
Aero/HS products
Net Sales:
Net sales	196.0	198.5
Automotive Extrusions
Net Sales:
Net sales	47.4	53.0
GE products
Net Sales:
Net sales	121.7	130.0
Other products
Net Sales:
Net sales	$ 4.2	13.7
Previously Reported [Member] | Transferred at Point in Time
Net Sales:
Revenue from Contract with Customer, Excluding Assessed Tax		131.8
Previously Reported [Member] | Transferred over Time
Net Sales:
Revenue from Contract with Customer, Excluding Assessed Tax		$ 263.4