Segment and Geographical Area Information, Schedule of net sales by end market segment applications (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer
Net sales	$ 353.1	$ 332.8	$ 356.3	$ 355.3	$ 331.9	$ 320.6	$ 334.9	$ 343.2	$ 1,397.5	$ 1,330.6	$ 1,391.9
Aero/HS products
Revenue from External Customer
Net sales									653.7	675.4	695.5
Automotive Extrusions
Revenue from External Customer
Net sales									217.3	188.8	199.2
GE products
Revenue from External Customer
Net sales									476.2	420.1	426.1
Other products
Revenue from External Customer
Net sales									$ 50.3	$ 46.3	$ 71.1