Derivatives, Hedging Programs and Other Financial Instruments, Realized and Unrealized Gains (Losses) Table (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments Gain Loss [Line Items]
Cost of products sold, excluding depreciation and amortization and other items1	[1]	$ 323.3	$ 256.4	$ 983.4	$ 808.7
Cost of Sales
Derivative Instruments Gain Loss [Line Items]
Cost of products sold, excluding depreciation and amortization and other items1			267.2		822.7
Cost of Sales | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:		3.1	(0.3)	(0.9)	(0.2)
Cost of Sales | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:		0.0	3.9	0.0	13.6
Cost of Sales | Aluminum | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:		2.1	0.0	(1.2)	0.0
Cost of Sales | Aluminum | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:		0.0	(4.0)	0.0	(13.8)
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:		1.0	(0.3)	0.4	(0.2)
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:		0.0		(0.1)
Cost of Sales | Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:		0.0	0.2	0.0	0.3
Cost of Sales | Foreign Exchange Contract | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:		$ 0.0	(0.1)	$ 0.0	(0.1)
Gain (Loss) on Derivative Instruments
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:			10.8		14.0
Gain (Loss) on Derivative Instruments | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:			0.0		0.0
Gain (Loss) on Derivative Instruments | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:			10.8		14.0
Gain (Loss) on Derivative Instruments | Aluminum | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:			0.0		0.0
Gain (Loss) on Derivative Instruments | Aluminum | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:			(10.6)		(15.3)
Gain (Loss) on Derivative Instruments | Alloy Metal Hedge | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:			0.0		0.0
Gain (Loss) on Derivative Instruments | Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:			(0.2)		1.3
Gain (Loss) on Derivative Instruments | Foreign Exchange Contract | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in income related to cash flow hedges:			$ 0.0		$ 0.0

[1] See Note 5 for discussion of our adoption of ASU 2017-12 (as defined in Note 1) and the related reclassification of amounts previously presented in the Statements of Consolidated Income within Unrealized (gain) loss on derivative instruments and now included within Cost of products sold, excluding depreciation and amortization and other items.