Segment and Geographical Area Information, Net Sales by End Market Segment Applications (Details) - Fabricated Products - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Net Sales:
Net sales	$ 355.3	$ 343.2
Aero/HS products
Net Sales:
Net sales	167.5	176.9
Automotive Extrusions
Net Sales:
Net sales	53.0	48.4
GE products
Net Sales:
Net sales	123.2	105.4
Other products
Net Sales:
Net sales	$ 11.6	$ 12.5