Segment and Geographical Area Information, Net Sales by End Market Segment Applications (Details) - Fabricated Products - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net Sales:
Net sales	$ 415.4	$ 356.3	$ 803.4	$ 711.6
Transferred at Point in Time
Net Sales:
Net sales	140.8		297.4
Transferred over Time
Net Sales:
Net sales	274.6		506.0
Aero/HS products
Net Sales:
Net sales	185.0	165.5	355.2	333.0
Automotive Extrusions
Net Sales:
Net sales	67.3	55.9	127.8	108.9
GE products
Net Sales:
Net sales	147.9	122.0	291.2	245.2
Other products
Net Sales:
Net sales	$ 15.2	$ 12.9	$ 29.2	$ 24.5