Fair Value Measurements, Fair Value Hierarchy Table (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financial Assets:
Cash and cash equivalents	$ 255.6	$ 49.8
Long-term available for sale securities	5.7	4.9
Total Assets	396.9	101.4
Financial Liabilities:
Convertible Notes	(238.4)	(203.0)
Total Liabilities	(545.3)	(278.2)
Commercial paper not included with cash and cash equivalents [Member]
Financial Assets:
Short-term investments	79.9
Aluminum | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	1.8	0.3
Financial Liabilities:
Derivative Liabilities	(1.4)	(7.8)
Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets	0.9	0.1
Financial Liabilities:
Derivative Liabilities		(0.1)
Natural Gas | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0.7
Financial Liabilities:
Derivative Liabilities	(1.4)	(1.3)
Natural Gas | Put option sales contracts
Financial Liabilities:
Derivative Liabilities	(1.6)	(5.6)
Electricity | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0.2
Financial Liabilities:
Derivative Liabilities	(0.3)	(1.8)
Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	52.1	46.3
Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	(58.6)	(53.9)
Level 1
Financial Assets:
Cash and cash equivalents	135.4	49.8
Long-term available for sale securities	0	0
Total Assets	135.4	49.8
Financial Liabilities:
Convertible Notes	(238.4)	(203.0)
Total Liabilities	(482.0)	(203.0)
Level 1 | Commercial paper not included with cash and cash equivalents [Member]
Financial Assets:
Short-term investments	0
Level 1 | Aluminum | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities	0	0
Level 1 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities		0
Level 1 | Natural Gas | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	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	120.2	0
Long-term available for sale securities	5.7	4.9
Total Assets	260.6	51.5
Financial Liabilities:
Convertible Notes	0	0
Total Liabilities	(63.3)	(75.1)
Level 2 | Commercial paper not included with cash and cash equivalents [Member]
Financial Assets:
Short-term investments	79.9
Level 2 | Aluminum | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	1.8	0.3
Financial Liabilities:
Derivative Liabilities	(1.4)	(7.8)
Level 2 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities		0
Level 2 | Natural Gas | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0.7
Financial Liabilities:
Derivative Liabilities	(1.4)	(1.3)
Level 2 | Natural Gas | Put option sales contracts
Financial Liabilities:
Derivative Liabilities	(1.6)	(5.6)
Level 2 | Electricity | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0.2
Financial Liabilities:
Derivative Liabilities	(0.3)	(1.8)
Level 2 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	52.1	46.3
Level 2 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	(58.6)	(53.9)
Level 3
Financial Assets:
Cash and cash equivalents	0	0
Long-term available for sale securities	0	0
Total Assets	0.9	0.1
Financial Liabilities:
Convertible Notes	0	0
Total Liabilities	0	(0.1)
Level 3 | Commercial paper not included with cash and cash equivalents [Member]
Financial Assets:
Short-term investments	0
Level 3 | Aluminum | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities	0	0
Level 3 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets	0.9	0.1
Financial Liabilities:
Derivative Liabilities		(0.1)
Level 3 | Natural Gas | Fixed priced purchase contracts
Financial Assets:
Derivative Assets	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
Nichols Promissory Note
Financial Liabilities:
Promissory Note		(4.7)
Nichols Promissory Note | Level 1
Financial Liabilities:
Promissory Note		0
Nichols Promissory Note | Level 2
Financial Liabilities:
Promissory Note		(4.7)
Nichols Promissory Note | Level 3
Financial Liabilities:
Promissory Note		0
Senior Notes
Financial Liabilities:
Senior Notes	(243.6)
Senior Notes | Level 1
Financial Liabilities:
Senior Notes	(243.6)
Senior Notes | Level 2
Financial Liabilities:
Senior Notes	0
Senior Notes | Level 3
Financial Liabilities:
Senior Notes	$ 0