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Fair Value Measurements (Details) - Recurring [Member] - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
ASSETS:
Cash equivalents	$ 13.3	$ 54.2
Trust assets	1.5	0.0
Derivative instruments	14.8	15.1
TOTAL	29.6	69.3
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Contingent obligation – net	0.0	0.0
Derivative instruments	111.0	16.7
TOTAL	111.0	16.7
Level 1 [Member]
ASSETS:
Cash equivalents	13.3	54.2
Trust assets	1.5	0.0
Derivative instruments	0.0	0.0
TOTAL	14.8	54.2
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Contingent obligation – net	0.0	0.0
Derivative instruments	0.0	0.0
TOTAL	0.0	0.0
Level 2 [Member]
ASSETS:
Cash equivalents	0.0	0.0
Trust assets	0.0	0.0
Derivative instruments	14.8	12.1
TOTAL	14.8	12.1
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Contingent obligation – net	0.0	0.0
Derivative instruments	104.8	16.7
TOTAL	104.8	16.7
Level 3 [Member]
ASSETS:
Cash equivalents	0.0	0.0
Trust assets	0.0	0.0
Derivative instruments	0.0	3.0
TOTAL	0.0	3.0
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Contingent obligation – net	0.0	0.0
Derivative instruments	6.2	0.0
TOTAL	$ 6.2	$ 0.0

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Fair values as of the balance sheet date of all primary aluminum sales contracts that meet the criteria of being accounted for as liability derivative instruments that do not qualify for normal purchases/normal sales exclusion.

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Fair value as of the balance sheet date of all trust assets as of the balance sheet date.

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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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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, 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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Fair value of obligations measured on a recurring basis.

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Fair value of financial and nonfinancial obligations.

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