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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from External Customer
Net sales	$ 368.7	$ 374.9	$ 375.3	$ 395.2	$ 389.4	$ 393.1	$ 415.4	$ 388.0	$ 1,514.1	$ 1,585.9	$ 1,397.5
Aero/HS products
Revenue from External Customer
Net sales									803.2	739.4	653.7
Automotive Extrusions
Revenue from External Customer
Net sales									190.5	239.3	217.3
GE products
Revenue from External Customer
Net sales									480.1	546.0	476.2
Other products
Revenue from External Customer
Net sales									40.3	61.2	$ 50.3
Transferred at Point in Time
Revenue from External Customer
Net sales									866.9	912.7
Transferred over Time
Revenue from External Customer
Net sales									$ 647.2	673.2
Previously reported | Transferred at Point in Time
Revenue from External Customer
Net sales										543.0
Previously reported | Transferred over Time
Revenue from External Customer
Net sales										$ 1,042.9