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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)			$ (14.0)	$ (16.9)
Cost of Sales | Aluminum | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss	$ (4.0)	$ 0.4	(13.8)	4.1
Cost of Sales | Natural Gas | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss	0.2	0.9	0.3	4.2
Cost of Sales | Alloy Metal Hedge | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss	(0.3)	0.0	(0.2)	0.0
Cost of Sales | Foreign Exchange Contract | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss	(0.1)	0.0	(0.1)	0.0
Cost of Sales | Operational Risk Hedges | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss	(4.2)	1.3	(13.8)	8.3
Gain (Loss) on Derivative Instruments | Aluminum | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)	(10.6)	(1.7)	(15.3)	(11.7)
Gain (Loss) on Derivative Instruments | Natural Gas | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)	(0.2)	(0.3)	1.3	(5.2)
Gain (Loss) on Derivative Instruments | Operational Risk Hedges | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)	$ (10.8)	$ (2.0)	$ (14.0)	$ (16.9)