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, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	$ (19.1)	$ 4.4	$ (1.9)
Unrealized gains (losses)	16.0	(25.9)	(5.6)
Aluminum
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	(9.0)	9.6	(0.6)
Unrealized gains (losses)	10.1	(26.5)	3.6
Natural Gas
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	(6.7)	(5.2)	(1.3)
Unrealized gains (losses)	4.3	(1.6)	(4.4)
Electricity
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	(3.4)	0	0
Unrealized gains (losses)	0.8	(1.8)	0
Foreign Currency
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	0	0	0.1
Call Options | Hedges Relating to the Convertible Notes
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	9.0	(2.1)	17.0
Bifurcated Conversion Feature | Hedges Relating to the Convertible Notes
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	$ (8.2)	$ 6.1	$ (21.9)