Business, Product and Geographical Area Information and Concentration of Risk - Schedule of net sales by end market segment applications (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from External Customer
Revenues	$ 389.4	$ 393.1	$ 415.4	$ 388.0	$ 353.1	$ 332.8	$ 356.3	$ 355.3	$ 1,585.9	$ 1,397.5	$ 1,330.6
Aero/HS products
Revenue from External Customer
Revenues									739.4	653.7	675.4
Automotive Extrusions
Revenue from External Customer
Revenues									239.3	217.3	188.8
GE products
Revenue from External Customer
Revenues									546.0	476.2	420.1
Other products
Revenue from External Customer
Revenues									61.2	$ 50.3	$ 46.3
Transferred at Point in Time
Revenue from External Customer
Revenues									543.0
Transferred over Time
Revenue from External Customer
Revenues									$ 1,042.9