Derivative Financial Instruments and Related Hedging Programs, Realized and Unrealized Gains and Losses Table (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	$ (5.7)		$ 0.5		$ (16.2)		$ 8.4
Unrealized gains (losses)	12.2		(12.7)		16.4		(19.8)
Aluminum
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	(3.3)		1.6		(7.7)		11.9
Unrealized gains (losses)	8.3		(14.8)		8.8		(21.4)
Natural Gas
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	(1.6)		(1.1)		(5.5)		(3.5)
Unrealized gains (losses)	3.0		(0.9)		4.7		0.6
Electricity
Summary Of Realized And Unrealized Gains Losses
Realized (losses) gains	(0.8)		0		(3.0)		0
Unrealized gains (losses)	1.0		(1.1)		1.8		(1.2)
Hedges Relating to the Convertible Notes
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	(0.1)	[1]	4.1	[1]	1.1	[1]	2.2	[1]
Hedges Relating to the Convertible Notes | Call Options relating to the Convertible Notes
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	10.2		(16.6)		5.8		(10.2)
Hedges Relating to the Convertible Notes | Cash conversion feature of the Convertible Notes
Summary Of Realized And Unrealized Gains Losses
Unrealized gains (losses)	$ (10.3)		$ 20.7		$ (4.7)		$ 12.4

[1] See “Derivative Financial Instruments” in Note 1 for a discussion of accounting policy for such instruments.