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Derivatives, Hedging Programs and Other Financial Instruments, Realized and Unrealized Gains (Losses) Table (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments Gain Loss [Line Items]
Cost of products sold, excluding depreciation and amortization and other items	$ 303.5	$ 343.4	$ 618.6	$ 660.1
Cost of Sales
Derivative Instruments Gain Loss [Line Items]
Cost of products sold, excluding depreciation and amortization and other items		343.4		660.1
Cost of Sales \| Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:	4.7	(3.9)	8.9	(4.0)
Cost of Sales \| Aluminum \| Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:	4.5	(3.6)	8.7	(3.3)
Cost of Sales \| Alloy Metal Hedge \| Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:	0.1	(0.2)	0.2	(0.6)
Cost of Sales \| Natural Gas \| Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:	$ 0.1	$ (0.1)	$ 0.0	$ (0.1)