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Derivatives and Fair Value Measurements - Narrative (Details) $ in Millions | 6 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2021 AUD ($) t | Sep. 30, 2021 t | Dec. 31, 2021 $ / $ | Jun. 30, 2021 USD ($) | Dec. 31, 2020 USD ($)
Foreign currency option contracts |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Notional amount | $ 580,000,000.0 |  |  |  |
Coal trading contracts |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Derivative notional amount (in tonnes) | t | 2,800,000 |  |  |  |
Diesel Fuel Hedge Contracts |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Notional amount | $ 0 |  |  |  |
Forecast | Foreign currency option contracts |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Derivative, exchange rate floor (in dollars per share) | $ / $ |  |  | 0.80 |  |
Derivative, exchange rate cap (in dollars per share) | $ / $ |  |  | 0.81 |  |
Forecast | Coal trading contracts |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Derivative notional amount (in tonnes) | t |  | 400,000 |  |  |
Coal Trading |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Derivative, initial margin posted |  |  |  | $ 14.4 | $ 3.0

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