Fair Value Measurements, Fair Value Hierarchy Table (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Financial Assets:
Derivative Asset	$ 5.6	$ 1.6
Deferred compensation plan assets	8.3	7.3
Financial Liabilities:
Derivative Liability	(3.2)	(16.2)
Fair Value, Measurements, Recurring
Financial Assets:
Cash and cash equivalents	72.9	72.5
Deferred compensation plan assets	8.3	7.3
Total Assets	287.8	111.4
Financial Liabilities:
Total Liabilities	(395.5)	(223.5)
Fair Value, Measurements, Recurring | Senior Notes Due 2020
Financial Liabilities:
Senior Notes		(207.3)
Fair Value, Measurements, Recurring | Senior Notes Due 2024
Financial Liabilities:
Senior Notes	(392.3)
Fair Value, Measurements, Recurring | Debt securities
Financial Assets:
Short-term investments	201.0	30.0
Fair Value, Measurements, Recurring | Level 1
Financial Assets:
Cash and cash equivalents	26.8	40.3
Deferred compensation plan assets	0.0	0.0
Total Assets	26.8	40.3
Financial Liabilities:
Total Liabilities	(392.3)	(207.3)
Fair Value, Measurements, Recurring | Level 1 | Senior Notes Due 2020
Financial Liabilities:
Senior Notes		(207.3)
Fair Value, Measurements, Recurring | Level 1 | Senior Notes Due 2024
Financial Liabilities:
Senior Notes	(392.3)
Fair Value, Measurements, Recurring | Level 1 | Debt securities
Financial Assets:
Short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 2
Financial Assets:
Cash and cash equivalents	46.1	32.2
Deferred compensation plan assets	8.3	7.3
Total Assets	260.9	70.2
Financial Liabilities:
Total Liabilities	(2.4)	(15.9)
Fair Value, Measurements, Recurring | Level 2 | Senior Notes Due 2020
Financial Liabilities:
Senior Notes		0.0
Fair Value, Measurements, Recurring | Level 2 | Senior Notes Due 2024
Financial Liabilities:
Senior Notes	0.0
Fair Value, Measurements, Recurring | Level 2 | Debt securities
Financial Assets:
Short-term investments	201.0	30.0
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	0.1	0.9
Financial Liabilities:
Total Liabilities	(0.8)	(0.3)
Fair Value, Measurements, Recurring | Level 3 | Senior Notes Due 2020
Financial Liabilities:
Senior Notes		0.0
Fair Value, Measurements, Recurring | Level 3 | Senior Notes Due 2024
Financial Liabilities:
Senior Notes	0.0
Fair Value, Measurements, Recurring | Level 3 | Debt securities
Financial Assets:
Short-term investments	0.0	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Aluminum
Financial Assets:
Derivative Asset	0.1	0.9
Financial Liabilities:
Derivative Liability	(0.8)	(0.3)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Aluminum | Purchase
Financial Assets:
Derivative Asset	5.1	0.3
Financial Liabilities:
Derivative Liability	(0.4)	(8.9)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Aluminum | Sales
Financial Assets:
Derivative Asset		0.2
Financial Liabilities:
Derivative Liability		(0.1)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Natural Gas | Purchase
Financial Assets:
Derivative Asset	0.4
Financial Liabilities:
Derivative Liability	(2.0)	(6.7)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 1 | Aluminum
Financial Assets:
Derivative Asset	0.0	0.0
Financial Liabilities:
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 1 | Aluminum | Purchase
Financial Assets:
Derivative Asset	0.0	0.0
Financial Liabilities:
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 1 | Aluminum | Sales
Financial Assets:
Derivative Asset		0.0
Financial Liabilities:
Derivative Liability		0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 1 | Natural Gas | Purchase
Financial Assets:
Derivative Asset	0.0
Financial Liabilities:
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 2 | Aluminum
Financial Assets:
Derivative Asset	0.0	0.0
Financial Liabilities:
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 2 | Aluminum | Purchase
Financial Assets:
Derivative Asset	5.1	0.3
Financial Liabilities:
Derivative Liability	(0.4)	(8.9)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 2 | Aluminum | Sales
Financial Assets:
Derivative Asset		0.2
Financial Liabilities:
Derivative Liability		(0.1)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 2 | Natural Gas | Purchase
Financial Assets:
Derivative Asset	0.4
Financial Liabilities:
Derivative Liability	(2.0)	(6.7)
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 3 | Aluminum | Purchase
Financial Assets:
Derivative Asset	0.0	0.0
Financial Liabilities:
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 3 | Aluminum | Sales
Financial Assets:
Derivative Asset		0.0
Financial Liabilities:
Derivative Liability		0.0
Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 3 | Natural Gas | Purchase
Financial Assets:
Derivative Asset	0.0
Financial Liabilities:
Derivative Liability	0.0	0.0
Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Foreign Currency | Purchase
Financial Liabilities:
Derivative Liability		(0.2)
Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 1 | Foreign Currency | Purchase
Financial Liabilities:
Derivative Liability		0.0
Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 2 | Foreign Currency | Purchase
Financial Liabilities:
Derivative Liability		(0.2)
Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 3 | Foreign Currency | Purchase
Financial Liabilities:
Derivative Liability		0.0
Income Approach Valuation Technique | Not Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 3 | Aluminum
Financial Assets:
Derivative Asset	0.1	0.9
Financial Liabilities:
Derivative Liability	$ (0.8)	(0.3)
Call Option | Fair Value, Measurements, Recurring | Aluminum
Financial Assets:
Derivative Asset		0.2
Call Option | Fair Value, Measurements, Recurring | Level 1 | Aluminum
Financial Assets:
Derivative Asset		0.0
Call Option | Fair Value, Measurements, Recurring | Level 2 | Aluminum
Financial Assets:
Derivative Asset		0.2
Call Option | Fair Value, Measurements, Recurring | Level 3 | Aluminum
Financial Assets:
Derivative Asset		$ 0.0