Derivative Financial Instruments and Related Hedging Programs, Realized and Unrealized Gains and Losses Table (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary Of Realized And Unrealized Gains Losses
Unrealized Gain (Loss) on Derivatives	$ (3.4)	$ (6.8)	$ 3.9
Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	(34.6)	7.8	(6.5)
Unrealized Gain (Loss) on Derivatives	(3.4)	(6.8)	3.9
Operational Risk Hedges | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains Losses
Unrealized Gain (Loss) on Derivatives	(3.4)	(10.4)	0.7
Aluminum | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	(27.3)	6.9	(5.5)
Unrealized Gain (Loss) on Derivatives	(4.6)	(2.6)	(3.1)
Natural Gas | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	(5.4)	1.0	(1.8)
Unrealized Gain (Loss) on Derivatives	(0.5)	(6.0)	2.6
Electricity | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	(1.9)	(0.1)	0.8
Unrealized Gain (Loss) on Derivatives	1.7	(1.8)	1.1
Foreign Currency | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains Losses
Unrealized Gain (Loss) on Derivatives	0.0	0.0	0.1
Foreign Currency | Operational Risk Hedges | Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains Losses
Unrealized Gain (Loss) on Derivatives		0.0	0.0
Hedges Relating to the Convertible Notes | Option Assets relating to the Convertible Notes3 | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains Losses
Unrealized Gain (Loss) on Derivatives	0.0	5.2	24.2
Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature of the Convertible Notes | Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains Losses
Unrealized Gain (Loss) on Derivatives	0.0	$ (1.6)	$ (21.0)
Long [Member] | Foreign Currency | Swap contracts | Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains Losses
Unrealized Gain (Loss) on Derivatives	$ (0.3)