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Derivatives and Fair Value Measurements - Textuals (Details) T in Millions, $ in Millions, $ in Millions | Dec. 31, 2019 T | Sep. 30, 2019 $ / $ | Mar. 31, 2019 AUD ($) T | Mar. 31, 2019 USD ($) T | Dec. 31, 2018 USD ($)
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Foreign currency option contracts |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Derivative, Notional Amount |  |  | $ 975.0 |  |
Coal |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Derivative, Nonmonetary Notional Amount | T |  |  | 3.7 | 3.7 |
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 | 2.2 |  |  |  |
Scenario, Forecast | Minimum | Foreign currency option contracts |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Derivative, Exchange Rate Cap | $ / $ |  | 0.76 |  |  |
Scenario, Forecast | Maximum | Foreign currency option contracts |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Derivative, Exchange Rate Cap | $ / $ |  | 0.77 |  |  |
Coal Trading |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |
Derivative, Fair Value, Amount Offset Against Collateral, Net |  |  |  | $ 18.2 | $ 21.8
Derivative, Collateral, Right to Reclaim Cash |  |  |  | 15.5 | 16.7
Receivables from Brokers-Dealers and Clearing Organizations |  |  |  | 6.0 | 2.2
Potential collateralization that may be requested by counterparties due to a material adverse event |  |  |  | $ 0.1 | $ 1.3
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 |  |  | 43.00% | 43.00% |
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 |  |  | 57.00% | 57.00% |

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- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226016-175313 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959 + Details
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