Fair Value Measurements, Fair Value Hierarchy Table (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Financial Assets:
Derivative Asset	$ 0.3	$ 85.7
Deferred compensation plan assets	7.0	7.3
Financial Liabilities:
Derivative Liability	(19.3)	(96.8)
Recurring
Financial Assets:
Cash and cash equivalents	82.6	177.7
Deferred compensation plan assets	7.0	7.3
Total Assets	119.9	384.7
Financial Liabilities:
Convertible Notes		(263.3)
Total Liabilities	(252.3)	(604.6)
Recurring | Senior Notes
Financial Liabilities:
Senior Notes	(233.0)	(244.5)
Recurring | Debt securities
Financial Assets:
Short-term investments	30.0	114.0
Recurring | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset	0.1
Recurring | Level 1
Financial Assets:
Cash and cash equivalents	37.1	29.5
Deferred compensation plan assets	0.0	0.0
Total Assets	37.1	29.5
Financial Liabilities:
Convertible Notes		(263.3)
Total Liabilities	(233.0)	(507.8)
Recurring | Level 1 | Senior Notes
Financial Liabilities:
Senior Notes	(233.0)	(244.5)
Recurring | Level 1 | Debt securities
Financial Assets:
Short-term investments	0.0	0.0
Recurring | Level 1 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset	0.0
Recurring | Level 2
Financial Assets:
Cash and cash equivalents	45.5	148.2
Deferred compensation plan assets	7.0	7.3
Total Assets	82.7	354.2
Financial Liabilities:
Convertible Notes		0.0
Total Liabilities	(17.3)	(96.8)
Recurring | Level 2 | Senior Notes
Financial Liabilities:
Senior Notes	0.0	0.0
Recurring | Level 2 | Debt securities
Financial Assets:
Short-term investments	30.0	114.0
Recurring | Level 2 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset	0.0
Recurring | Level 3
Financial Assets:
Cash and cash equivalents	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total Assets	0.1	1.0
Financial Liabilities:
Convertible Notes		0.0
Total Liabilities	(2.0)	0.0
Recurring | Level 3 | Senior Notes
Financial Liabilities:
Senior Notes	0.0	0.0
Recurring | Level 3 | Debt securities
Financial Assets:
Short-term investments	0.0	0.0
Not Designated as Hedging Instrument | Recurring | Aluminum | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(10.3)	(4.2)
Not Designated as Hedging Instrument | Recurring | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Asset	0.2
Not Designated as Hedging Instrument | Recurring | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset		1.0
Financial Liabilities:
Derivative Liability	(2.0)
Not Designated as Hedging Instrument | Recurring | Natural Gas | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(6.4)	(6.2)
Not Designated as Hedging Instrument | Recurring | Electricity | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(0.4)	(1.7)
Not Designated as Hedging Instrument | Recurring | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Asset		84.7
Not Designated as Hedging Instrument | Recurring | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liability		(84.7)
Not Designated as Hedging Instrument | Recurring | Level 1 | Aluminum | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Recurring | Level 1 | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Asset	0.0
Not Designated as Hedging Instrument | Recurring | Level 1 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset		0.0
Financial Liabilities:
Derivative Liability	0.0
Not Designated as Hedging Instrument | Recurring | Level 1 | Natural Gas | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Recurring | Level 1 | Electricity | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Recurring | Level 1 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Asset		0.0
Not Designated as Hedging Instrument | Recurring | Level 1 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liability		0.0
Not Designated as Hedging Instrument | Recurring | Level 2 | Aluminum | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(10.3)	(4.2)
Not Designated as Hedging Instrument | Recurring | Level 2 | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Asset	0.2
Not Designated as Hedging Instrument | Recurring | Level 2 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset		0.0
Financial Liabilities:
Derivative Liability	0.0
Not Designated as Hedging Instrument | Recurring | Level 2 | Natural Gas | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(6.4)	(6.2)
Not Designated as Hedging Instrument | Recurring | Level 2 | Electricity | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(0.4)	(1.7)
Not Designated as Hedging Instrument | Recurring | Level 2 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Asset		84.7
Not Designated as Hedging Instrument | Recurring | Level 2 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liability		(84.7)
Not Designated as Hedging Instrument | Recurring | Level 3 | Aluminum | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Recurring | Level 3 | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Asset	0.0
Not Designated as Hedging Instrument | Recurring | Level 3 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset		1.0
Not Designated as Hedging Instrument | Recurring | Level 3 | Natural Gas | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Recurring | Level 3 | Electricity | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Recurring | Level 3 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Asset		0.0
Not Designated as Hedging Instrument | Recurring | Level 3 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liability		$ 0.0
Designated as Hedging Instrument [Member] | Recurring | Foreign Currency Member | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(0.2)
Designated as Hedging Instrument [Member] | Recurring | Level 1 | Foreign Currency Member | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0
Designated as Hedging Instrument [Member] | Recurring | Level 2 | Foreign Currency Member | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(0.2)
Designated as Hedging Instrument [Member] | Recurring | Level 3 | Foreign Currency Member | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0
Income Approach Valuation Technique | Level 3 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset	0.1
Income Approach Valuation Technique | Not Designated as Hedging Instrument | Level 3 | Aluminum | Midwest premium swap contracts
Financial Liabilities:
Derivative Liability	$ (2.0)