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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

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-Publisher FASB
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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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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.

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The net gain (loss) recognized in earnings during the reporting period due to the ineffectiveness of price risk fair value hedges.

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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.

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