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Derivatives, Hedging Programs and Other Financial Instruments - Summary of (Gain) Loss Associated with Derivative Contracts (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
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 cashflow hedges are recorded | $ 2,691.1 | $ 2,754.9 | $ 3,180.2
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 cashflow hedges are recorded | Total of income and expense line items presented in our Statements of Consolidated Income (loss) in which the effects of cashflow hedges are recorded | Total of income and expense line items presented in our Statements of Consolidated Income (loss) in which the effects of cashflow hedges are recorded
Not Designated as Hedging Instrument |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |
Loss (gain) recognized in our Statements of Consolidated Income (Loss) | $ 5.0 | $ 0.1 | $ 0.9
Not Designated as Hedging Instrument | Alloying Metals |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |
Loss (gain) recognized in our Statements of Consolidated Income (Loss) |  | 0.1 | (0.5)
Loss (gain) recognized in our Statements of Consolidated Income (Loss) - Unrealized mark-to-market loss |  |  | 1.4
Not Designated as Hedging Instrument | Electricity |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |
Loss (gain) recognized in our Statements of Consolidated Income (Loss) | 5.0 |  |
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 cashflow hedges are recorded | 2,691.1 | 2,754.9 | 3,180.2
Cash Flow Hedges | Designated as Hedging Instrument |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |
Loss (gain) recognized in our Statements of Consolidated Income (Loss) | (0.9) | 12.9 | (10.5)
Cash Flow Hedges | Designated as Hedging Instrument | Aluminum |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |
Loss (gain) recognized in our Statements of Consolidated Income (Loss) | (1.4) | 12.8 | 7.4
Cash Flow Hedges | Designated as Hedging Instrument | Alloying Metals |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |
Loss (gain) recognized in our Statements of Consolidated Income (Loss) | (1.1) |  |
Cash Flow Hedges | Designated as Hedging Instrument | Natural Gas |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |
Loss (gain) recognized in our Statements of Consolidated Income (Loss) | 1.0 | $ 0.1 | (6.6)
Cash Flow Hedges | Designated as Hedging Instrument | Electricity |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |
Loss (gain) recognized in our Statements of Consolidated Income (Loss) | $ 0.6 |  | $ (11.3)

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