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Restructuring Costs (Details) $ in Millions	3 Months Ended	12 Months Ended
Restructuring Costs (Details) $ in Millions	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) employee	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 4.5	$ 0.0	$ 0.0
Restructuring and other charges	$ 4.5	$ 4.5	$ 0.0	$ 0.0
Flat Rolled Products
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Number of Positions Eliminated \| employee		70