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Derivatives and Fair Value Measurements - Textuals (Details) T in Millions, $ in Millions, $ in Millions | 3 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Mar. 31, 2020 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2020 T | Sep. 30, 2020 $ / $ | Mar. 31, 2020 AUD ($) T | Mar. 31, 2020 USD ($) T | Dec. 31, 2019 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 |  |  |  |  | $ 550.0 |  |
Coal |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |
Derivative, Nonmonetary Notional Amount | T |  |  |  |  | 1.6 | 1.6 |
Diesel Fuel Hedge Contracts |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |
Derivative, Notional Amount |  |  |  |  | $ 0.0 |  |
Scenario, Forecast | Coal |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |
Derivative, Nonmonetary Notional Amount | T |  |  | 1.3 |  |  |  |
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 | $ / $ |  |  |  | 0.75 |  |  |
Coal Trading |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |
Derivative, initial margin posted |  |  |  |  |  | $ 5.6 | $ 7.9
Derivatives, amount posted in excess of initial margin requirements |  |  |  |  |  | 2.8 | 1.3
Derivative, variation margin |  |  |  |  |  | 16.8 | 22.1
Potential collateralization that may be requested by counterparties due to a material adverse event |  |  |  |  |  | $ 0.8 | $ 0.0

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- Definition Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226016-175313 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109980867&loc=d3e41678-113959 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 + Details
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