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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments Gain Loss [Line Items]
Total amounts of income and expense line items presented in the statements of consolidated income in which the effects of hedges are recorded	[1]	$ 343.4	$ 289.6	$ 660.1	$ 552.3
Cost of Sales
Derivative Instruments Gain Loss [Line Items]
Total amounts of income and expense line items presented in the statements of consolidated income in which the effects of hedges are recorded			277.7		555.5
Cost of Sales | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in income related to cash flow hedges:		(3.9)	0.2	(4.0)	0.1
Cost of Sales | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in income related to cash flow hedges:		0.0	(5.1)	0.0	(9.7)
Cost of Sales | Aluminum | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in income related to cash flow hedges:		(3.6)	0.0	(3.3)	0.0
Cost of Sales | Aluminum | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in income related to cash flow hedges:		0.0	(5.2)	0.0	(9.8)
Cost of Sales | Alloy Metal Hedge | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in income related to cash flow hedges:		(0.2)	0.2	(0.6)	0.1
Cost of Sales | Natural Gas | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in income related to cash flow hedges:		(0.1)		(0.1)
Cost of Sales | Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss 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]
Total amounts of income and expense line items presented in the statements of consolidated income in which the effects of hedges are recorded			11.9		(3.2)
Gain (Loss) on Derivative Instruments | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss 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]
(Gain) loss recognized in income related to cash flow hedges:			11.9		(3.2)
Gain (Loss) on Derivative Instruments | Aluminum | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss 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]
(Gain) loss recognized in income related to cash flow hedges:			11.6		(4.7)
Gain (Loss) on Derivative Instruments | Alloy Metal Hedge | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss 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]
(Gain) loss recognized in income related to cash flow hedges:			$ 0.3		$ 1.5

[1] See Note 4 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 loss (gain) on derivative instruments and now included within Cost of products sold, excluding depreciation and amortization and other items.