Fair Value Measurements (Details 1) (Fair Value, Measurements, Recurring [Member], USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Derivative assets
Derivative assets	$ 73.8	$ 79.1
Derivative liabilities
Derivative Liabilities	(79.5)	(78.4)
Call option purchase contracts [Member] | Aluminum Commodity [Member] | Level 2 [Member]
Derivative assets
Derivative assets	4.8	9.3
Call option purchase contracts [Member] | Aluminum Commodity [Member]
Derivative assets
Derivative assets	4.8	9.3
Call option purchase contracts [Member] | Natural Gas [Member] | Level 2 [Member]
Derivative assets
Derivative assets	0.1	0.3
Call option purchase contracts [Member] | Natural Gas [Member]
Derivative assets
Derivative assets	0.1	0.3
Aluminum Commodity [Member] | Fixed priced purchase contracts [Member] | Level 2 [Member]
Derivative assets
Derivative assets	11.8	18.2
Derivative liabilities
Derivative Liabilities	(0.9)	(0.4)
Aluminum Commodity [Member] | Fixed priced purchase contracts [Member]
Derivative assets
Derivative assets	11.8	18.2
Derivative liabilities
Derivative Liabilities	(0.9)	(0.4)
Natural Gas [Member] | Fixed priced purchase contracts [Member] | Level 2 [Member]
Derivative assets
Derivative assets		0.1
Derivative liabilities
Derivative Liabilities	(0.3)	(0.5)
Natural Gas [Member] | Fixed priced purchase contracts [Member]
Derivative assets
Derivative assets		0.1
Derivative liabilities
Derivative Liabilities	(0.3)	(0.5)
Electricity [Member] | Fixed priced purchase contracts [Member] | Level 2 [Member]
Derivative liabilities
Derivative Liabilities	(0.1)
Electricity [Member] | Fixed priced purchase contracts [Member]
Derivative liabilities
Derivative Liabilities	(0.1)
Aluminum Commodity [Member] | Fixed priced sales contracts [Member] | Level 2 [Member]
Derivative assets
Derivative assets	0.2
Derivative liabilities
Derivative Liabilities	(1.9)	(3.4)
Aluminum Commodity [Member] | Fixed priced sales contracts [Member]
Derivative assets
Derivative assets	0.2
Derivative liabilities
Derivative Liabilities	(1.9)	(3.4)
Midwest premium swap contracts [Member] | Aluminum Commodity [Member] | Level 3 [Member]
Derivative assets
Derivative assets	0.9	0.2
Derivative liabilities
Derivative Liabilities		(0.1)
Midwest premium swap contracts [Member] | Aluminum Commodity [Member]
Derivative assets
Derivative assets	0.9	0.2
Derivative liabilities
Derivative Liabilities		(0.1)
Put option purchase contracts [Member] | Aluminum Commodity [Member] | Level 2 [Member]
Derivative assets
Derivative assets		0.1
Put option purchase contracts [Member] | Aluminum Commodity [Member]
Derivative assets
Derivative assets		0.1
Put option purchase contracts [Member] | Natural Gas [Member] | Level 2 [Member]
Derivative assets
Derivative assets	1.2	2.5
Put option purchase contracts [Member] | Natural Gas [Member]
Derivative assets
Derivative assets	1.2	2.5
Call Option [Member] | Hedges Relating to Notes [Member] | Level 2 [Member]
Derivative assets
Derivative assets	54.8	48.4
Call Option [Member] | Hedges Relating to Notes [Member]
Derivative assets
Derivative assets	54.8	48.4
Call option sales contracts [Member] | Aluminum Commodity [Member] | Level 2 [Member]
Derivative liabilities
Derivative Liabilities	(4.7)	(9.3)
Call option sales contracts [Member] | Aluminum Commodity [Member]
Derivative liabilities
Derivative Liabilities	(4.7)	(9.3)
Put option sales contracts [Member] | Aluminum Commodity [Member] | Level 2 [Member]
Derivative liabilities
Derivative Liabilities		(0.1)
Put option sales contracts [Member] | Aluminum Commodity [Member]
Derivative liabilities
Derivative Liabilities		(0.1)
Put option sales contracts [Member] | Natural Gas [Member] | Level 2 [Member]
Derivative liabilities
Derivative Liabilities	(3.3)	(4.6)
Put option sales contracts [Member] | Natural Gas [Member]
Derivative liabilities
Derivative Liabilities	(3.3)	(4.6)
Bifurcated Conversion Feature [Member] | Hedges Relating to Notes [Member] | Level 2 [Member]
Derivative liabilities
Derivative Liabilities	(68.3)	(60.0)
Bifurcated Conversion Feature [Member] | Hedges Relating to Notes [Member]
Derivative liabilities
Derivative Liabilities	(68.3)	(60.0)
Level 2 [Member]
Derivative assets
Derivative assets	72.9	78.9
Derivative liabilities
Derivative Liabilities	(79.5)	(78.3)
Level 3 [Member]
Derivative assets
Derivative assets	0.9	0.2
Derivative liabilities
Derivative Liabilities		$ (0.1)