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Fair Value Measurements - Significant Unobservable Inputs (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2022 USD ($) t MT | Dec. 31, 2021 USD ($) MT | Dec. 31, 2020 USD ($)
LME Swap |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Fair value | $ (4.6) | $ (5.1) | $ 2.9
Open position to offset fixed prices | t | 111,741 |  |
FX Swap |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Fair value | $ 0.0 | $ (0.2) | 0.1
FX Swap | Level 3 |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Derivative asset, percentage | 0.0858 |  |
FX Swap | Discounted Cash Flow | Level 3 |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Derivative asset, percentage |  | 0.0858 |
Nordpool Power Price Swap |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Fair value | $ 0.0 | $ 0.2 | $ 0.0
Nordpool Power Price Swap | Level 3 |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Fair value | $ 0.0 | 0.2 |
Derivative asset, percentage | 0.0858 |  |
Casthouse |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Fair value | $ 0.0 | 0.0 |
Contingent obligation |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Fair value | $ 0.0 | $ 0.0 |
Contingent obligation | Level 3 | Minimum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Open position to offset fixed prices | MT | 0 | 14,000 |
Contingent obligation | Level 3 | Maximum |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Open position to offset fixed prices | MT |  | 15,000 |

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