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Derivatives and Fair Value Measurements Financial Instruments Measured on a Recurring Basis (Details) - Successor - Fair Value, Measurements, Recurring - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | $ 11.8 | $ (5.0)
Level 1 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 0.0 | (3.0)
Level 2 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 1.8 | (2.0)
Level 3 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 10.0 | 0.0
Foreign currency option contracts |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 1.2 | 4.2
Foreign currency option contracts | Level 1 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 0.0 | 0.0
Foreign currency option contracts | Level 2 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 1.2 | 4.2
Foreign currency option contracts | Level 3 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 0.0 | 0.0
Coal contracts related to forecasted sales |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | (21.2) | (40.2)
Coal contracts related to forecasted sales | Level 1 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 0.0 | 0.0
Coal contracts related to forecasted sales | Level 2 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | (21.2) | (40.2)
Coal contracts related to forecasted sales | Level 3 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 0.0 | 0.0
Coal trading contracts |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 21.8 | 31.0
Coal trading contracts | Level 1 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 0.0 | (3.0)
Coal trading contracts | Level 2 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 21.8 | 34.0
Coal trading contracts | Level 3 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 0.0 | $ 0.0
Equity Securities |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 10.0 |
Equity Securities | Level 1 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 0.0 |
Equity Securities | Level 2 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | 0.0 |
Equity Securities | Level 3 |  |
Derivative [Line Items] |  |
Derivative Assets (Liabilities), at Fair Value, Net | $ 10.0 |

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 + Details
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