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Derivatives and Fair Value Measurements - Textuals (Details) T in Millions, $ in Millions | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2019 USD ($) T | Jun. 30, 2018 USD ($) | Jun. 30, 2019 USD ($) T | Jun. 30, 2018 USD ($) | Mar. 31, 2020 $ / $ | Dec. 31, 2019 T | Jun. 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 |  |  |  |  |  |  | $ 1,000.0 |
Coal |  |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |
Derivative, nonmonetary notional amount | T | 3.2 |  | 3.2 |  |  |  | 3.2 |
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 |  |  |  |  |  | 1.7 |  |
Scenario, Forecast | Minimum | Foreign currency option contracts |  |  |  |  |  |  |  |
Derivatives, Fair Value [Line Items] |  |  |  |  |  |  |  |
Derivative, exchange rate cap | $ / $ |  |  |  |  | 0.74 |  |  |
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, initial margin posted | $ 15,300,000 |  | $ 15,300,000 |  |  |  |  | $ 16,700,000
Derivatives, amount posted in excess of initial margin requirements | 300,000 |  | 300,000 |  |  |  |  | 2,200,000
Derivative, variation margin | 46,100,000 |  | 46,100,000 |  |  |  |  | 21,800,000
Potential collateralization that may be requested by counterparties due to a material adverse event | $ 0 |  | $ 0 |  |  |  |  | $ 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 | 62.00% |  | 62.00% |  |  |  | 62.00% |
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 | 38.00% |  | 38.00% |  |  |  | 38.00% |

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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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