Derivative Financial Instruments and Related Hedging Programs, Realized and Unrealized Gains and Losses Table (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	$ (6.8)	$ 3.9	$ 16.0
Not Designated as Hedging Instrument
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	7.8	(6.5)	(19.1)
Unrealized gains (losses)	(6.8)	3.9	16.0
Not Designated as Hedging Instrument | Aluminum
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	6.9	(5.5)	(9.0)
Unrealized gains (losses)	(2.6)	(3.1)	10.1
Not Designated as Hedging Instrument | Natural Gas
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	1.0	(1.8)	(6.7)
Unrealized gains (losses)	(6.0)	2.6	4.3
Not Designated as Hedging Instrument | Electricity
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	(0.1)	0.8	(3.4)
Unrealized gains (losses)	(1.8)	1.1	0.8
Not Designated as Hedging Instrument | Foreign Currency
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	0	0.1	0
Not Designated as Hedging Instrument | Operational Risk Hedges [Member]
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	(10.4)	0.7	15.2
Not Designated as Hedging Instrument | Option Assets relating to the Convertible Notes | Hedges Relating to the Convertible Notes
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	5.2	24.2	9.0
Not Designated as Hedging Instrument | Bifurcated Conversion Feature of the Convertible Notes | Hedges Relating to the Convertible Notes
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	$ (1.6)	$ (21.0)	$ (8.2)