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Derivatives and Fair Value Measurements - Textuals (Details) T in Millions | 3 Months Ended | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2019 USD ($) T | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) T | Sep. 30, 2018 USD ($) | Dec. 31, 2020 T | Jun. 30, 2020 $ / $ | Dec. 31, 2019 T | Sep. 30, 2019 AUD ($) T | Dec. 31, 2018 USD ($)
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |  |
Transfers between fair value levels 1, 2, and 3 | $ 0 | $ 0 | $ 0 | $ 0 |  |  |  |  |
Foreign currency option contracts |  |  |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  |  |  |  |  |  | $ 925,000,000.0 |
Coal |  |  |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Nonmonetary Notional Amount | T | 2.8 |  | 2.8 |  |  |  |  | 2.8 |
Diesel Fuel Hedge Contracts |  |  |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  |  |  |  |  |  |  | $ 0 |
Scenario, Forecast | Coal |  |  |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |  |
Derivative, Nonmonetary Notional Amount | T |  |  |  |  | 1.6 |  | 0.9 |  |
Scenario, Forecast | Minimum | Foreign currency option contracts |  |  |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |  |
Derivative, exchange rate cap | $ / $ |  |  |  |  |  | 0.73 |  |  |
Scenario, Forecast | Maximum | Foreign currency option contracts |  |  |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |  |
Derivative, exchange rate cap | $ / $ |  |  |  |  |  | 0.76 |  |  |
Coal Trading |  |  |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |  |
Derivative, initial margin posted | $ 12,800,000 |  | $ 12,800,000 |  |  |  |  |  | $ 16,700,000
Derivatives, amount posted in excess of initial margin requirements | 200,000 |  | 200,000 |  |  |  |  |  | 2,200,000
Derivative, variation margin | 30,000,000.0 |  | 30,000,000.0 |  |  |  |  |  | 21,800,000
Potential collateralization that may be requested by counterparties due to a material adverse event | $ 100,000 |  | $ 100,000 |  |  |  |  |  | $ 1,300,000
Coal Trading Positions [Member] | External Credit Rating, Investment Grade [Member] | Coal Trading | Credit Concentration Risk [Member] |  |  |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |  |
Credit exposure related to Coal Trading activities | 57.00% |  | 57.00% |  |  |  |  | 57.00% |
Coal Trading Positions [Member] | Non Rated [Member] | Coal Trading | Credit Concentration Risk [Member] |  |  |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |  |
Credit exposure related to Coal Trading activities | 43.00% |  | 43.00% |  |  |  |  | 43.00% |

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