Fair Value Measurements, Fair Value Hierarchy Table (Details) (Recurring, USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financial Assets:
Cash and cash equivalents	$ 248.0	$ 273.4
Available for sale securities	5.8	5.6
Total Assets	402.8	422.5
Financial Liabilities:
Convertible Notes	(244.3)	(240.1)
Total Liabilities	(569.4)	(556.8)
Aluminum | Fixed priced purchase contracts
Financial Assets:
Derivative Assets		2.6
Financial Liabilities:
Derivative Liabilities	(2.4)	(0.5)
Aluminum | Fixed priced sales contracts
Financial Assets:
Derivative Assets	0.1
Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets	0.4	0.4
Natural Gas | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	1.0	0.2
Financial Liabilities:
Derivative Liabilities	(1.1)	(2.6)
Natural Gas | Put option sales contracts
Financial Liabilities:
Derivative Liabilities	(0.1)	(0.5)
Electricity | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0.6
Financial Liabilities:
Derivative Liabilities	(0.5)	(1.0)
Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	61.3	55.3
Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	(67.7)	(62.1)
Level 1
Financial Assets:
Cash and cash equivalents	82.9	107.9
Available for sale securities	0	0
Total Assets	82.9	107.9
Financial Liabilities:
Convertible Notes	(244.3)	(240.1)
Total Liabilities	(497.4)	(490.1)
Level 1 | Aluminum | Fixed priced purchase contracts
Financial Assets:
Derivative Assets		0
Financial Liabilities:
Derivative Liabilities	0	0
Level 1 | Aluminum | Fixed priced sales contracts
Financial Assets:
Derivative Assets	0
Level 1 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets	0	0
Level 1 | Natural Gas | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities	0	0
Level 1 | Natural Gas | Put option sales contracts
Financial Liabilities:
Derivative Liabilities	0	0
Level 1 | Electricity | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0
Financial Liabilities:
Derivative Liabilities	0	0
Level 1 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	0	0
Level 1 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	0	0
Level 2
Financial Assets:
Cash and cash equivalents	165.1	165.5
Available for sale securities	5.8	5.6
Total Assets	319.5	314.2
Financial Liabilities:
Convertible Notes	0	0
Total Liabilities	(72.0)	(66.7)
Level 2 | Aluminum | Fixed priced purchase contracts
Financial Assets:
Derivative Assets		2.6
Financial Liabilities:
Derivative Liabilities	(2.4)	(0.5)
Level 2 | Aluminum | Fixed priced sales contracts
Financial Assets:
Derivative Assets	0.1
Level 2 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets	0	0
Level 2 | Natural Gas | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	1.0	0.2
Financial Liabilities:
Derivative Liabilities	(1.1)	(2.6)
Level 2 | Natural Gas | Put option sales contracts
Financial Liabilities:
Derivative Liabilities	(0.1)	(0.5)
Level 2 | Electricity | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0.6
Financial Liabilities:
Derivative Liabilities	(0.5)	(1.0)
Level 2 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	61.3	55.3
Level 2 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	(67.7)	(62.1)
Level 3
Financial Assets:
Cash and cash equivalents	0	0
Available for sale securities	0	0
Total Assets	0.4	0.4
Financial Liabilities:
Convertible Notes	0	0
Total Liabilities	0	0
Level 3 | Aluminum | Fixed priced purchase contracts
Financial Assets:
Derivative Assets		0
Financial Liabilities:
Derivative Liabilities	0	0
Level 3 | Aluminum | Fixed priced sales contracts
Financial Assets:
Derivative Assets	0
Level 3 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets	0.4	0.4
Level 3 | Natural Gas | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities	0	0
Level 3 | Natural Gas | Put option sales contracts
Financial Liabilities:
Derivative Liabilities	0	0
Level 3 | Electricity | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0
Financial Liabilities:
Derivative Liabilities	0	0
Level 3 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	0	0
Level 3 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	0	0
Senior Notes
Financial Liabilities:
Senior Notes	(253.1)	(250.0)
Senior Notes | Level 1
Financial Liabilities:
Senior Notes	(253.1)	(250.0)
Senior Notes | Level 2
Financial Liabilities:
Senior Notes	0	0
Senior Notes | Level 3
Financial Liabilities:
Senior Notes	0	0
Short-term investments
Financial Assets:
Short-term investments	85.6	85.0
Short-term investments | Level 1
Financial Assets:
Short-term investments	0	0
Short-term investments | Level 2
Financial Assets:
Short-term investments	85.6	85.0
Short-term investments | Level 3
Financial Assets:
Short-term investments	0	0
Euro | Foreign Currency | Fixed priced purchase contracts
Financial Liabilities:
Derivative Liabilities	(0.2)
Euro | Level 1 | Foreign Currency | Fixed priced purchase contracts
Financial Liabilities:
Derivative Liabilities	0
Euro | Level 2 | Foreign Currency | Fixed priced purchase contracts
Financial Liabilities:
Derivative Liabilities	(0.2)
Euro | Level 3 | Foreign Currency | Fixed priced purchase contracts
Financial Liabilities:
Derivative Liabilities	$ 0