Derivative Financial Instruments and Related Hedging Programs, Realized and Unrealized Gains and Losses Table (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains:	$ (7.8)	$ 4.8	$ (10.5)	$ 7.9
Unrealized gains (losses)	0.6	(13.1)	4.2	(7.1)
Aluminum
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains:	(4.2)	5.8	(4.4)	10.3
Unrealized gains (losses)	(4.7)	(9.7)	0.5	(6.6)
Natural Gas
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains:	(2.1)	(1.0)	(3.9)	(2.4)
Unrealized gains (losses)	3.0	0.3	1.8	1.5
Electricity
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains:	(1.5)	0	(2.2)	0
Unrealized gains (losses)	1.6	(0.1)	0.7	(0.1)
Hedges Relating to the Convertible Notes | Call Options relating to the Convertible Notes
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	4.4	8.4	(4.4)	6.4
Hedges Relating to the Convertible Notes | Cash conversion feature of the Convertible Notes
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	$ (3.7)	$ (12.0)	$ 5.6	$ (8.3)