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Derivatives and Fair Value Measurements Pre-Tax Gains and Losses on Hedging Derivatives (Details) - Designated as Hedging Instrument - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Apr. 01, 2017 | Dec. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2016
Successor |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income |  | $ 12.3 | $ 103.7 |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net |  | 30.1 | 83.7 |
Unrealized (loss) gain recognized in income on derivatives |  | (17.8) | 20.0 |
Successor | Foreign currency option contracts |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income |  | 1.8 | (9.1) |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net |  | 3.3 | (8.4) |
Unrealized (loss) gain recognized in income on derivatives |  | (1.5) | (0.7) |
Successor | Coal contracts related to forecasted sales |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income |  | 12.1 | 115.7 |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net |  | 35.1 | 97.4 |
Unrealized (loss) gain recognized in income on derivatives |  | (23.0) | 18.3 |
Successor | Coal trading contracts |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income |  | (1.6) | (2.9) |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net |  | (8.3) | (5.3) |
Unrealized (loss) gain recognized in income on derivatives |  | $ 6.7 | $ 2.4 |
Predecessor |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income | $ 3.8 |  |  | $ (327.0)
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net | 1.4 |  |  | 67.5
Unrealized gain recognized in income on derivatives | 30.0 |  |  | (78.4)
Loss reclassified from other comprehensive loss into income | (27.6) |  |  | (231.7)
Loss reclassified from other comprehensive income into income (ineffective portion) |  |  |  | (84.4)
Predecessor | Foreign currency option contracts |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income | (16.6) |  |  | (179.3)
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net | 0.0 |  |  | 0.0
Unrealized gain recognized in income on derivatives | 0.0 |  |  | 0.0
Loss reclassified from other comprehensive loss into income | (16.6) |  |  | (145.6)
Loss reclassified from other comprehensive income into income (ineffective portion) |  |  |  | (33.7)
Predecessor | Coal contracts related to forecasted sales |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income | 29.2 |  |  | 5.6
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net | 12.7 |  |  | 45.4
Unrealized gain recognized in income on derivatives | 16.5 |  |  | (39.8)
Loss reclassified from other comprehensive loss into income | 0.0 |  |  | 0.0
Loss reclassified from other comprehensive income into income (ineffective portion) |  |  |  | 0.0
Predecessor | Coal trading contracts |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income | 2.2 |  |  | (16.5)
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net | (11.3) |  |  | 22.1
Unrealized gain recognized in income on derivatives | 13.5 |  |  | (38.6)
Loss reclassified from other comprehensive loss into income | 0.0 |  |  | 0.0
Loss reclassified from other comprehensive income into income (ineffective portion) |  |  |  | 0.0
Predecessor | Commodity swap contracts |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Total (loss) gain recognized in income | (11.0) |  |  | (136.8)
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net | 0.0 |  |  | 0.0
Unrealized gain recognized in income on derivatives | 0.0 |  |  | 0.0
Loss reclassified from other comprehensive loss into income | $ (11.0) |  |  | (86.1)
Loss reclassified from other comprehensive income into income (ineffective portion) |  |  |  | $ (50.7)

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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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- Definition The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959 + Details
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- Definition The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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- Definition The net gain (loss) recognized in earnings during the reporting period due to the ineffectiveness of price risk fair value hedges. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986 + Details
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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 + Details
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