Derivatives, Hedging Programs and Other Financial Instruments - Summary of Gain Associated with Derivative Contracts (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Derivative Instruments Gain Loss [Line Items]
Total of income and expense line items presented in our Statements of Consolidated Income in which the effects of cash flow hedges are recorded	$ 865.9	$ 262.5
Cost of Products Sold \| Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Gain recognized in our Statements of Consolidated Income	(1.6)	(0.3)
Cost of Products Sold \| Not Designated as Hedging Instrument \| Alloying Metals
Derivative Instruments Gain Loss [Line Items]
Gain recognized in our Statements of Consolidated Income	(0.6)
Gain recognized in our Statements of Consolidated Income Unrealized Gain	(1.0)	(0.3)
Cash Flow Hedges
Derivative Instruments Gain Loss [Line Items]
Total of income and expense line items presented in our Statements of Consolidated Income in which the effects of cash flow hedges are recorded	865.9	262.5
Cash Flow Hedges \| Cost of Products Sold \| Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Gain recognized in our Statements of Consolidated Income	(19.9)	(3.8)
Cash Flow Hedges \| Cost of Products Sold \| Designated as Hedging Instrument \| Electricity
Derivative Instruments Gain Loss [Line Items]
Gain recognized in our Statements of Consolidated Income	(0.3)
Cash Flow Hedges \| Cost of Products Sold \| Designated as Hedging Instrument \| Aluminum
Derivative Instruments Gain Loss [Line Items]
Gain recognized in our Statements of Consolidated Income	(18.7)	(2.9)
Cash Flow Hedges \| Cost of Products Sold \| Designated as Hedging Instrument \| Alloying Metals
Derivative Instruments Gain Loss [Line Items]
Gain recognized in our Statements of Consolidated Income		$ (0.9)
Cash Flow Hedges \| Cost of Products Sold \| Designated as Hedging Instrument \| Natural Gas
Derivative Instruments Gain Loss [Line Items]
Gain recognized in our Statements of Consolidated Income	$ (0.9)

The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 924
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-URI http://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(a),(d))
-URI http://asc.fasb.org/extlink&oid=123367319&loc=SL114868664-224227

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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5618551-113959

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
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