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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)			$ (3.2)	$ (14.9)
Operational Risk Hedges | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)	$ 11.9	$ (10.9)	(3.2)	(14.9)
Cost of Sales [Member] | Alloy Metal Hedge [Member] | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss	0.2	0.0	0.1	0.0
Cost of Sales [Member] | Aluminum | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss	(5.2)	1.0	(9.8)	3.7
Cost of Sales [Member] | Operational Risk Hedges | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss	(4.9)	2.6	(9.6)	7.0
Cost of Sales [Member] | Natural Gas | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Realized (gain) loss	0.1	1.6	0.1	3.3
Gain (Loss) on Derivative Instruments | Aluminum | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)	11.6	(6.9)	(4.7)	(10.0)
Gain (Loss) on Derivative Instruments | Natural Gas | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains (Losses)
Unrealized loss (gain)	$ 0.3	$ (4.0)	$ 1.5	$ (4.9)