Derivatives, Hedging Programs and Other Financial Instruments, Realized and Unrealized Gains (Losses) Table (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments Gain Loss [Line Items]
Cost of Goods Sold	[1]	$ 316.7	$ 262.7
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)		0.0	(15.1)
Cost of Sales
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss			(0.1)
Cost of Sales | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss		(0.1)	0.0
Cost of Sales | Aluminum
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss		0.3	0.0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments			4.6
Cost of Sales | Aluminum | Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss		0.0
Cost of Sales | Aluminum | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss			0.0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments			16.3
Cost of Sales | Alloy Metal Hedge
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss			(0.1)
Cost of Sales | Alloy Metal Hedge | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss		(0.4)	0.0
Gain (Loss) on Derivative Instruments | Aluminum
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)			4.6
Gain (Loss) on Derivative Instruments | Aluminum | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)		0.0	15.1
Gain (Loss) on Derivative Instruments | Natural Gas Member
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)			0.0
Gain (Loss) on Derivative Instruments | Natural Gas Member | Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)		$ 0.0
Gain (Loss) on Derivative Instruments | Natural Gas Member | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)			1.2
Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Cost of Goods Sold			(15.1)
Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Cost of Goods Sold			$ 277.8

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