Segment and Geographical Area Information, Net Sales by End Market Segment Applications (Details) - Fabricated Products - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Sales:
Net sales	$ 332.8	$ 320.6	$ 1,044.4	$ 998.7
Aero/HS products
Net Sales:
Net sales	150.2	156.1	483.2	500.7
Automotive Extrusions
Net Sales:
Net sales	52.7	46.5	161.6	143.6
GE products
Net Sales:
Net sales	115.9	105.6	361.1	318.3
Other products
Net Sales:
Net sales	$ 14.0	$ 12.4	$ 38.5	$ 36.1