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Derivatives, Hedging Programs and Other Financial Instruments - Summary of (Gain) Loss Associated with Derivative Contracts (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Derivative Instruments Gain Loss [Line Items]
Total of income and expense line items presented in our Statements of Consolidated Income (Loss) in which the effects of cash flow hedges are recorded	$ 671.8	$ 665.2	$ 2,005.2	$ 2,114.7
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Total of income and expense line items presented in our Statements of Consolidated Income (Loss) in which the effects of cash flow hedges are recorded	Total of income and expense line items presented in our Statements of Consolidated Income (Loss) in which the effects of cash flow hedges are recorded	Total of income and expense line items presented in our Statements of Consolidated Income (Loss) in which the effects of cash flow hedges are recorded	Total of income and expense line items presented in our Statements of Consolidated Income (Loss) in which the effects of cash flow hedges are recorded
Loss (gain) recognized in our Statements of Consolidated Income - Unrealized mark-to-market (gain) loss			$ 2.2	$ (0.2)
Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income	$ 2.0	$ (0.2)	4.2	(0.1)
Not Designated as Hedging Instrument \| Alloying Metals
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income		0.1		0.1
Loss (gain) recognized in our Statements of Consolidated Income - Unrealized mark-to-market (gain) loss		(0.3)		(0.2)
Not Designated as Hedging Instrument \| Electricity
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income	2.0		2.0
Loss (gain) recognized in our Statements of Consolidated Income - Unrealized mark-to-market (gain) loss			2.0
Reclassification from AOCI due to forecasted transactions probable of not occurring			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 (Loss) in which the effects of cash flow hedges are recorded	671.8	665.2	2,005.2	2,114.7
Cash Flow Hedges \| Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income	1.5	4.5	1.6	10.2
Cash Flow Hedges \| Designated as Hedging Instrument \| Aluminum
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income	1.6	4.4	0.9	10.1
Cash Flow Hedges \| Designated as Hedging Instrument \| Alloying Metals
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income	(0.3)		(0.8)
Cash Flow Hedges \| Designated as Hedging Instrument \| Foreign exchange contracts
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income	(0.1)		(0.1)
Cash Flow Hedges \| Designated as Hedging Instrument \| Natural Gas
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income	$ 0.3	$ 0.1	1.0	$ 0.1
Cash Flow Hedges \| Designated as Hedging Instrument \| Electricity
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income			$ 0.6