Fair Value Measurements, Fair Value Hierarchy Table (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financial Assets:
Derivative Asset	$ 95.1us-gaap_DerivativeAssets	$ 85.7us-gaap_DerivativeAssets
Deferred compensation plan assets	7.1us-gaap_DeferredCompensationPlanAssets	7.3us-gaap_DeferredCompensationPlanAssets
Financial Liabilities:
Derivative Liability	(110.7)us-gaap_DerivativeLiabilities	(96.8)us-gaap_DerivativeLiabilities
Recurring
Financial Assets:
Cash and cash equivalents	220.9	177.7
Deferred compensation plan assets	7.1	7.3
Total Assets	353.1	384.7
Financial Liabilities:
Convertible Notes	(273.8)	(263.3)
Total Liabilities	(629.9)	(604.6)
Recurring \| Senior Notes
Financial Liabilities:
Senior Notes	(245.4)	(244.5)
Recurring \| Debt securities
Financial Assets:
Short-term investments	30.0	114.0
Recurring \| Aluminum \| Fixed priced contracts \| Purchase
Financial Assets:
Derivative Asset	0.1
Financial Liabilities:
Derivative Liability	(3.8)	(4.2)
Recurring \| Aluminum \| Midwest premium swap contracts
Financial Assets:
Derivative Asset	0.1	1.0
Financial Liabilities:
Derivative Liability	(3.7)
Recurring \| Natural Gas \| Fixed priced contracts \| Purchase
Financial Liabilities:
Derivative Liability	(7.0)	(6.2)
Recurring \| Electricity \| Fixed priced contracts \| Purchase
Financial Liabilities:
Derivative Liability	(1.3)	(1.7)
Recurring \| Hedges Relating to the Convertible Notes \| Call Options
Financial Assets:
Derivative Asset	94.9	84.7
Recurring \| Hedges Relating to the Convertible Notes \| Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liability	(94.9)	(84.7)
Recurring \| Level 1
Financial Assets:
Cash and cash equivalents	204.9	29.5
Deferred compensation plan assets	0	0
Total Assets	299.8	29.5
Financial Liabilities:
Convertible Notes	(273.8)	(263.3)
Total Liabilities	(614.1)	(507.8)
Recurring \| Level 1 \| Senior Notes
Financial Liabilities:
Senior Notes	(245.4)	(244.5)
Recurring \| Level 1 \| Debt securities
Financial Assets:
Short-term investments	0	0
Recurring \| Level 1 \| Aluminum \| Fixed priced contracts \| Purchase
Financial Assets:
Derivative Asset	0
Financial Liabilities:
Derivative Liability	0	0
Recurring \| Level 1 \| Aluminum \| Midwest premium swap contracts
Financial Assets:
Derivative Asset	0	0
Financial Liabilities:
Derivative Liability	0
Recurring \| Level 1 \| Natural Gas \| Fixed priced contracts \| Purchase
Financial Liabilities:
Derivative Liability	0	0
Recurring \| Level 1 \| Electricity \| Fixed priced contracts \| Purchase
Financial Liabilities:
Derivative Liability	0	0
Recurring \| Level 1 \| Hedges Relating to the Convertible Notes \| Call Options
Financial Assets:
Derivative Asset	94.9	0
Recurring \| Level 1 \| Hedges Relating to the Convertible Notes \| Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liability	(94.9)	0
Recurring \| Level 2
Financial Assets:
Cash and cash equivalents	16.0	148.2
Deferred compensation plan assets	7.1	7.3
Total Assets	53.2	354.2
Financial Liabilities:
Convertible Notes	0	0
Total Liabilities	(12.1)	(96.8)
Recurring \| Level 2 \| Senior Notes
Financial Liabilities:
Senior Notes	0	0
Recurring \| Level 2 \| Debt securities
Financial Assets:
Short-term investments	30.0	114.0
Recurring \| Level 2 \| Aluminum \| Fixed priced contracts \| Purchase
Financial Assets:
Derivative Asset	0.1
Financial Liabilities:
Derivative Liability	(3.8)	(4.2)
Recurring \| Level 2 \| Aluminum \| Midwest premium swap contracts
Financial Assets:
Derivative Asset	0	0
Financial Liabilities:
Derivative Liability	0
Recurring \| Level 2 \| Natural Gas \| Fixed priced contracts \| Purchase
Financial Liabilities:
Derivative Liability	(7.0)	(6.2)
Recurring \| Level 2 \| Electricity \| Fixed priced contracts \| Purchase
Financial Liabilities:
Derivative Liability	(1.3)	(1.7)
Recurring \| Level 2 \| Hedges Relating to the Convertible Notes \| Call Options
Financial Assets:
Derivative Asset	0	84.7
Recurring \| Level 2 \| Hedges Relating to the Convertible Notes \| Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liability	0	(84.7)
Recurring \| Level 3
Financial Assets:
Cash and cash equivalents	0	0
Deferred compensation plan assets	0	0
Total Assets	0.1	1.0
Financial Liabilities:
Convertible Notes	0	0
Total Liabilities	(3.7)	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 Asset	0
Financial Liabilities:
Derivative Liability	0	0
Recurring \| Level 3 \| Aluminum \| Midwest premium swap contracts
Financial Assets:
Derivative Asset		1.0
Recurring \| Level 3 \| Natural Gas \| Fixed priced contracts \| Purchase
Financial Liabilities:
Derivative Liability	0	0
Recurring \| Level 3 \| Electricity \| Fixed priced contracts \| Purchase
Financial Liabilities:
Derivative Liability	0	0
Recurring \| Level 3 \| Hedges Relating to the Convertible Notes \| Call Options
Financial Assets:
Derivative Asset	0	0
Recurring \| Level 3 \| Hedges Relating to the Convertible Notes \| Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liability	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock.

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

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