Segment and Geographical Area Information, Net Sales by End Market Segment Applications (Details) - Fabricated Products - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net Sales:
Net sales	$ 388.0	$ 355.3
Transferred at Point in Time
Net Sales:
Net sales	156.6
Transferred over Time
Net Sales:
Net sales	231.4
Aero/HS products
Net Sales:
Net sales	170.2	167.5
Automotive Extrusions
Net Sales:
Net sales	60.5	53.0
GE products
Net Sales:
Net sales	143.3	123.2
Other products
Net Sales:
Net sales	$ 14.0	$ 11.6