Fair Value Measurements, Fair Value Hierarchy Table (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Financial Assets:
Derivative Assets	$ 84.3	$ 58.5
Deferred compensation plan assets	6.3	5.6
Financial Liabilities:
Derivative Liabilities	(94.8)	(66.7)
Recurring
Financial Assets:
Cash and cash equivalents	224.6	273.4
Deferred compensation plan assets	6.3	5.6
Total Assets	431.1	422.5
Financial Liabilities:
Convertible Notes	(263.3)	(240.1)
Total Liabilities	(607.1)	(556.8)
Recurring | Senior Notes
Financial Liabilities:
Senior Notes	(249.0)	(250.0)
Recurring | Debt securities
Financial Assets:
Short-term investments	115.9	85.0
Recurring | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets		0.4
Recurring | Aluminum | Midwest premium swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	(0.8)
Recurring | Aluminum | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0.4	2.6
Financial Liabilities:
Derivative Liabilities	(1.7)	(0.5)
Recurring | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Assets	0.1
Recurring | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets		0.2
Financial Liabilities:
Derivative Liabilities	(2.0)	(2.6)
Recurring | Natural Gas | Option contracts | Sales
Financial Liabilities:
Derivative Liabilities	(0.1)	(0.5)
Recurring | Electricity | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0.6
Financial Liabilities:
Derivative Liabilities	(1.3)	(1.0)
Recurring | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	83.2	55.3
Recurring | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	(88.9)	(62.1)
Recurring | Level 1
Financial Assets:
Cash and cash equivalents	89.5	107.9
Deferred compensation plan assets	0	0
Total Assets	89.5	107.9
Financial Liabilities:
Convertible Notes	(263.3)	(240.1)
Total Liabilities	(512.3)	(490.1)
Recurring | Level 1 | Senior Notes
Financial Liabilities:
Senior Notes	(249.0)	(250.0)
Recurring | Level 1 | Debt securities
Financial Assets:
Short-term investments	0	0
Recurring | Level 1 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets		0
Recurring | Level 1 | Aluminum | Midwest premium swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	0
Recurring | Level 1 | Aluminum | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 1 | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Assets	0
Recurring | Level 1 | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets		0
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 1 | Natural Gas | Option contracts | Sales
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 1 | Electricity | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 1 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	0	0
Recurring | Level 1 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 2
Financial Assets:
Cash and cash equivalents	135.1	165.5
Deferred compensation plan assets	6.3	5.6
Total Assets	341.6	314.2
Financial Liabilities:
Convertible Notes	0	0
Total Liabilities	(94.0)	(66.7)
Recurring | Level 2 | Senior Notes
Financial Liabilities:
Senior Notes	0	0
Recurring | Level 2 | Debt securities
Financial Assets:
Short-term investments	115.9	85.0
Recurring | Level 2 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets		0
Recurring | Level 2 | Aluminum | Midwest premium swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	0
Recurring | Level 2 | Aluminum | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0.4	2.6
Financial Liabilities:
Derivative Liabilities	(1.7)	(0.5)
Recurring | Level 2 | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Assets	0.1
Recurring | Level 2 | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets		0.2
Financial Liabilities:
Derivative Liabilities	(2.0)	(2.6)
Recurring | Level 2 | Natural Gas | Option contracts | Sales
Financial Liabilities:
Derivative Liabilities	(0.1)	(0.5)
Recurring | Level 2 | Electricity | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0.6
Financial Liabilities:
Derivative Liabilities	(1.3)	(1.0)
Recurring | Level 2 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	83.2	55.3
Recurring | Level 2 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	(88.9)	(62.1)
Recurring | Level 3
Financial Assets:
Cash and cash equivalents	0	0
Deferred compensation plan assets	0	0
Total Assets	0	0.4
Financial Liabilities:
Convertible Notes	0	0
Total Liabilities	(0.8)	0
Recurring | Level 3 | Senior Notes
Financial Liabilities:
Senior Notes	0	0
Recurring | Level 3 | Debt securities
Financial Assets:
Short-term investments	0	0
Recurring | Level 3 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets		0.4
Recurring | Level 3 | Aluminum | Midwest premium swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	(0.8)
Recurring | Level 3 | Aluminum | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 3 | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Assets	0
Recurring | Level 3 | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets		0
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 3 | Natural Gas | Option contracts | Sales
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 3 | Electricity | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 3 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	0	0
Recurring | Level 3 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	$ 0	$ 0