Derivatives, Hedging Programs and Other Financial Instruments - Summary of (Gain) Loss Associated with Derivative Contracts (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
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	$ 262.5	$ 286.6
Total of income and expense line items presented in our Statements of Consolidated Income in which the effects of fair value hedges are recorded	13.5	13.2
Cost of Products Sold \| Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Gain recognized in our Statements of Consolidated Income – Unrealized gain	(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 – Unrealized gain	(0.3)
Depreciation and Amortization \| Designated as Hedging Instrument \| Foreign Exchange Contract
Derivative Instruments Gain Loss [Line Items]
Foreign exchange contracts		0.2
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	262.5	286.6
Cash Flow Hedges \| Cost of Products Sold \| Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income	(3.8)	7.0
Cash Flow Hedges \| Cost of Products Sold \| Designated as Hedging Instrument \| Electricity
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income		0.2
Cash Flow Hedges \| Cost of Products Sold \| Designated as Hedging Instrument \| Aluminum
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income	(2.9)	5.7
Cash Flow Hedges \| Cost of Products Sold \| Designated as Hedging Instrument \| Alloying Metals
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income	$ (0.9)	0.6
Cash Flow Hedges \| Cost of Products Sold \| Designated as Hedging Instrument \| Natural Gas
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income		0.5
Fair Value 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 fair value hedges are recorded		13.2
Fair Value Hedges \| Designated as Hedging Instrument \| Foreign Exchange Contract
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income		0.2
Fair Value Hedges \| Depreciation and Amortization \| Designated as Hedging Instrument \| Foreign Exchange Contract
Derivative Instruments Gain Loss [Line Items]
Hedged item		(0.2)
Excluded component amortized from AOCI		$ 0.2

Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

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Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

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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.

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

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