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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net Sales:
Net sales	$ 356.3	$ 334.9	$ 711.6	$ 678.1
Aero/HS products
Net Sales:
Net sales	165.5	167.7	333.0	344.6
Automotive Extrusions
Net Sales:
Net sales	55.9	48.8	108.9	97.1
GE products
Net Sales:
Net sales	122.0	107.3	245.2	212.7
Other products
Net Sales:
Net sales	$ 12.9	$ 11.1	$ 24.5	$ 23.7