Fair Value Measurements, Fair Value Hierarchy Table (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Financial Assets:
Derivative Asset	$ 5.4	$ 1.6
Deferred compensation plan assets	7.8	7.3
Financial Liabilities:
Derivative Liability	(5.0)	(16.2)
Fair Value, Measurements, Recurring
Financial Assets:
Cash and cash equivalents	195.3	72.5
Deferred compensation plan assets	7.8	7.3
Total Assets	280.5	111.4
Financial Liabilities:
Total Liabilities	(388.4)	(223.5)
Fair Value, Measurements, Recurring | Senior Notes Due 2020 [Member]
Financial Liabilities:
Senior Notes		(207.3)
Fair Value, Measurements, Recurring | Senior Notes Due 2024 [Member]
Financial Liabilities:
Senior Notes	(383.4)
Fair Value, Measurements, Recurring | Debt securities
Financial Assets:
Short-term investments	72.0	30.0
Fair Value, Measurements, Recurring | Aluminum | Option on Securities [Member] | Purchase
Financial Assets:
Derivative Asset		0.2
Fair Value, Measurements, Recurring | Aluminum | Fixed priced contracts | Purchase
Financial Assets:
Derivative Asset	4.8	0.3
Financial Liabilities:
Derivative Liability		(8.9)
Fair Value, Measurements, Recurring | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Asset		0.2
Financial Liabilities:
Derivative Liability		(0.1)
Fair Value, Measurements, Recurring | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset		0.9
Fair Value, Measurements, Recurring | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Asset	0.6
Financial Liabilities:
Derivative Liability		(6.7)
Fair Value, Measurements, Recurring | Level 1
Financial Assets:
Cash and cash equivalents	50.6	40.3
Deferred compensation plan assets	0.0	0.0
Total Assets	50.6	40.3
Financial Liabilities:
Total Liabilities	(383.4)	(207.3)
Fair Value, Measurements, Recurring | Level 1 | Senior Notes Due 2020 [Member]
Financial Liabilities:
Senior Notes		(207.3)
Fair Value, Measurements, Recurring | Level 1 | Senior Notes Due 2024 [Member]
Financial Liabilities:
Senior Notes	(383.4)
Fair Value, Measurements, Recurring | Level 1 | Debt securities
Financial Assets:
Short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Aluminum | Option on Securities [Member] | Purchase
Financial Assets:
Derivative Asset		0.0
Fair Value, Measurements, Recurring | Level 1 | Aluminum | Fixed priced contracts | Purchase
Financial Assets:
Derivative Asset	0.0	0.0
Financial Liabilities:
Derivative Liability		0.0
Fair Value, Measurements, Recurring | Level 1 | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Asset		0.0
Financial Liabilities:
Derivative Liability		0.0
Fair Value, Measurements, Recurring | Level 1 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset		0.0
Fair Value, Measurements, Recurring | Level 1 | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Asset	0.0
Financial Liabilities:
Derivative Liability		0.0
Fair Value, Measurements, Recurring | Level 2
Financial Assets:
Cash and cash equivalents	144.7	32.2
Short-term investments		30.0
Deferred compensation plan assets	7.8	7.3
Total Assets	229.9	70.2
Financial Liabilities:
Total Liabilities	(3.6)	(15.9)
Fair Value, Measurements, Recurring | Level 2 | Senior Notes Due 2020 [Member]
Financial Liabilities:
Senior Notes		0.0
Fair Value, Measurements, Recurring | Level 2 | Senior Notes Due 2024 [Member]
Financial Liabilities:
Senior Notes	0.0
Fair Value, Measurements, Recurring | Level 2 | Debt securities
Financial Assets:
Short-term investments	72.0
Fair Value, Measurements, Recurring | Level 2 | Aluminum | Option on Securities [Member] | Purchase
Financial Assets:
Derivative Asset		0.2
Fair Value, Measurements, Recurring | Level 2 | Aluminum | Fixed priced contracts | Purchase
Financial Assets:
Derivative Asset	4.8	0.3
Financial Liabilities:
Derivative Liability		(8.9)
Fair Value, Measurements, Recurring | Level 2 | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Asset		0.2
Financial Liabilities:
Derivative Liability		(0.1)
Fair Value, Measurements, Recurring | Level 2 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset		0.0
Fair Value, Measurements, Recurring | Level 2 | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Asset	0.6
Financial Liabilities:
Derivative Liability		(6.7)
Fair Value, Measurements, Recurring | Level 3
Financial Assets:
Cash and cash equivalents	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total Assets	0.0	0.9
Financial Liabilities:
Total Liabilities	(1.4)	(0.3)
Fair Value, Measurements, Recurring | Level 3 | Senior Notes Due 2020 [Member]
Financial Liabilities:
Senior Notes		0.0
Fair Value, Measurements, Recurring | Level 3 | Senior Notes Due 2024 [Member]
Financial Liabilities:
Senior Notes	0.0
Fair Value, Measurements, Recurring | Level 3 | Debt securities
Financial Assets:
Short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Aluminum | Option on Securities [Member] | Purchase
Financial Assets:
Derivative Asset		0.0
Fair Value, Measurements, Recurring | Level 3 | Aluminum | Fixed priced contracts | Purchase
Financial Assets:
Derivative Asset	0.0	0.0
Financial Liabilities:
Derivative Liability		0.0
Fair Value, Measurements, Recurring | Level 3 | Aluminum | Fixed priced contracts | Sales
Financial Assets:
Derivative Asset		0.0
Financial Liabilities:
Derivative Liability		0.0
Fair Value, Measurements, Recurring | Level 3 | Natural Gas | Fixed priced contracts | Purchase
Financial Assets:
Derivative Asset	0.0
Financial Liabilities:
Derivative Liability		0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Aluminum | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(1.2)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Aluminum | Midwest premium swap contracts
Financial Liabilities:
Derivative Liability	(1.4)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Natural Gas | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(2.4)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 1 | Aluminum | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 1 | Aluminum | Midwest premium swap contracts
Financial Liabilities:
Derivative Liability	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 1 | Natural Gas | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 2 | Aluminum | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(1.2)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 2 | Aluminum | Midwest premium swap contracts
Financial Liabilities:
Derivative Liability	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 2 | Natural Gas | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	(2.4)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 3 | Aluminum | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 3 | Natural Gas | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability	0.0
Designated as Hedging Instrument [Member] | Fair Value, Measurements, Recurring | Foreign Currency Member | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability		(0.2)
Designated as Hedging Instrument [Member] | Fair Value, Measurements, Recurring | Level 1 | Foreign Currency Member | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability		0.0
Designated as Hedging Instrument [Member] | Fair Value, Measurements, Recurring | Level 2 | Foreign Currency Member | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability		(0.2)
Designated as Hedging Instrument [Member] | Fair Value, Measurements, Recurring | Level 3 | Foreign Currency Member | Fixed priced contracts | Purchase
Financial Liabilities:
Derivative Liability		0.0
Income Approach Valuation Technique | Fair Value, Measurements, Recurring | Aluminum | Midwest premium swap contracts
Financial Liabilities:
Derivative Liability		(0.3)
Income Approach Valuation Technique | Fair Value, Measurements, Recurring | Level 1 | Aluminum | Midwest premium swap contracts
Financial Liabilities:
Derivative Liability		0.0
Income Approach Valuation Technique | Fair Value, Measurements, Recurring | Level 2 | Aluminum | Midwest premium swap contracts
Financial Liabilities:
Derivative Liability		0.0
Income Approach Valuation Technique | Fair Value, Measurements, Recurring | Level 3 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset		0.9
Financial Liabilities:
Derivative Liability	$ (1.4)	$ (0.3)