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Derivatives, Hedging Programs and Other Financial Instruments - Summary of Loss (Gain) Associated with Derivative Contracts (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2024 | Mar. 31, 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 | $ 642.9 | $ 731.1
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
Not Designated as Hedging Instrument | Alloying Metals |  |
Derivative Instruments Gain Loss [Line Items] |  |
Gain recognized in our Statements of Consolidated Income - Unrealized gain |  | $ (0.1)
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 | $ 642.9 | 731.1
Cash Flow Hedges | Designated as Hedging Instrument |  |
Derivative Instruments Gain Loss [Line Items] |  |
Loss (gain) recognized in our Statements of Consolidated Income | 2.1 | 0.2
Cash Flow Hedges | Designated as Hedging Instrument | Aluminum |  |
Derivative Instruments Gain Loss [Line Items] |  |
Loss (gain) recognized in our Statements of Consolidated Income | 2.0 | 0.5
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 | $ (0.3)
Cash Flow Hedges | Designated as Hedging Instrument | Electricity |  |
Derivative Instruments Gain Loss [Line Items] |  |
Loss (gain) recognized in our Statements of Consolidated Income | $ (0.2) |

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