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Derivatives and Fair Value Measurements - Financial Instruments Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Derivative [Line Items] |  |
Net financial asset (liability) | $ 13.2 | $ 9.9
Level 1 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 0.0 | 0.0
Level 2 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 9.2 | 5.9
Level 3 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 4.0 | 4.0
Foreign currency option contracts |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 10.3 | 1.1
Foreign currency option contracts | Level 1 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 0.0 | 0.0
Foreign currency option contracts | Level 2 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 10.3 | 1.1
Foreign currency option contracts | Level 3 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 0.0 | 0.0
Coal contracts related to forecasted sales |  |
Derivative [Line Items] |  |
Net financial asset (liability) | (7.9) | 21.2
Coal contracts related to forecasted sales | Level 1 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 0.0 | 0.0
Coal contracts related to forecasted sales | Level 2 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | (7.9) | 21.2
Coal contracts related to forecasted sales | Level 3 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 0.0 | 0.0
Coal trading contracts |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 6.8 | (16.4)
Coal trading contracts | Level 1 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 0.0 | 0.0
Coal trading contracts | Level 2 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 6.8 | (16.4)
Coal trading contracts | Level 3 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 0.0 | 0.0
Equity Securities |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 4.0 | 4.0
Equity Securities | Level 1 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 0.0 | 0.0
Equity Securities | Level 2 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | 0.0 | 0.0
Equity Securities | Level 3 |  |
Derivative [Line Items] |  |
Net financial asset (liability) | $ 4.0 | $ 4.0

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- Definition Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=117331979&loc=d3e41228-113958 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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