Fair Value Measurements, Fair Value Hierarchy Table (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Financial Assets:
Derivative Assets	$ 101.8	$ 81.8
Deferred compensation plan assets	7.2	6.5
Financial Liabilities:
Derivative Liabilities	(102.5)	(86.1)
Recurring
Financial Assets:
Cash and cash equivalents	212.3	169.5
Deferred compensation plan assets	7.2	6.5
Total Assets	421.4	387.3
Financial Liabilities:
Convertible Notes	(280.8)	(260.0)
Total Liabilities	(631.1)	(601.5)
Recurring | Senior Notes
Financial Liabilities:
Senior Notes	(247.8)	(255.4)
Recurring | Debt securities
Financial Assets:
Short-term investments	100.1	129.5
Recurring | Aluminum | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0.5	0.1
Financial Liabilities:
Derivative Liabilities	(1.3)	(1.8)
Recurring | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets	1.5	1.1
Recurring | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0.2	0.5
Financial Liabilities:
Derivative Liabilities	(1.4)	(0.8)
Recurring | Electricity | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0.3	0.5
Financial Liabilities:
Derivative Liabilities	(0.5)	(0.4)
Recurring | Foreign Currency | Fixed priced contracts | Purchase | Euro
Financial Assets:
Derivative Assets		0.1
Recurring | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	99.3	79.5
Recurring | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	(99.3)	(83.1)
Recurring | Level 1
Financial Assets:
Cash and cash equivalents	35.4	57.7
Deferred compensation plan assets	0	0
Total Assets	35.4	57.7
Financial Liabilities:
Convertible Notes	(280.8)	(260.0)
Total Liabilities	(528.6)	(515.4)
Recurring | Level 1 | Senior Notes
Financial Liabilities:
Senior Notes	(247.8)	(255.4)
Recurring | Level 1 | Debt securities
Financial Assets:
Short-term investments	0	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 | Midwest premium swap contracts
Financial Assets:
Derivative Assets	0	0
Recurring | Level 1 | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 1 | Electricity | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 1 | Foreign Currency | Fixed priced contracts | Purchase | Euro
Financial Assets:
Derivative Assets		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	176.9	111.8
Deferred compensation plan assets		6.5
Total Assets	384.5	328.5
Financial Liabilities:
Convertible Notes	0	0
Total Liabilities	(102.5)	(86.1)
Recurring | Level 2 | Senior Notes
Financial Liabilities:
Senior Notes	0	0
Recurring | Level 2 | Debt securities
Financial Assets:
Short-term investments		129.5
Recurring | Level 2 | Aluminum | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0.5	0.1
Financial Liabilities:
Derivative Liabilities	(1.3)	(1.8)
Recurring | Level 2 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets	0	0
Recurring | Level 2 | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0.2	0.5
Financial Liabilities:
Derivative Liabilities	(1.4)	(0.8)
Recurring | Level 2 | Electricity | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0.3	0.5
Financial Liabilities:
Derivative Liabilities	(0.5)	(0.4)
Recurring | Level 2 | Foreign Currency | Fixed priced contracts | Purchase | Euro
Financial Assets:
Derivative Assets		0.1
Recurring | Level 2 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Assets	99.3	79.5
Recurring | Level 2 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities	(99.3)	(83.1)
Recurring | Level 3
Financial Assets:
Cash and cash equivalents	0	0
Deferred compensation plan assets	0	0
Total Assets	1.5	1.1
Financial Liabilities:
Convertible Notes	0	0
Total Liabilities	0	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 | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 3 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Assets	1.5	1.1
Recurring | Level 3 | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 3 | Electricity | Fixed priced contracts | Purchase
Financial Assets:
Derivative Assets	0	0
Financial Liabilities:
Derivative Liabilities	0	0
Recurring | Level 3 | Foreign Currency | Fixed priced contracts | Purchase | Euro
Financial Assets:
Derivative Assets		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