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Fair Value Measurements Fair Value - Schedule of Quantitative Information for Level 3 Derivative Contracts (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Asset	$ 85.7us-gaap_DerivativeAssets	$ 81.8us-gaap_DerivativeAssets
Fair Value, Measurements, Recurring [Member] \| Midwest premium swap contracts \| Aluminum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Asset	1.0	1.1
Fair Value, Measurements, Recurring [Member] \| Midwest premium swap contracts \| Aluminum \| Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Asset		1.1
Fair Value, Measurements, Recurring [Member] \| Midwest premium swap contracts \| Aluminum \| Income Approach Valuation Technique \| Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Asset	$ 1.0

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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