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, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	$ 3.8	$ 12.3	$ 63.6	$ 103.7
(Loss) gain realized in income on derivatives	1.4	30.1	11.8	83.7
Unrealized gain recognized in income on derivatives		(17.8)	51.8	20.0
Unrealized gain recognized in income on derivatives	30.0
Loss reclassified from other comprehensive loss into income	(27.6)
Foreign currency option contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	(16.6)	1.8	(3.7)	(9.1)
(Loss) gain realized in income on derivatives	0.0	3.3	(4.9)	(8.4)
Unrealized gain recognized in income on derivatives		(1.5)	1.2	(0.7)
Unrealized gain recognized in income on derivatives	0.0
Loss reclassified from other comprehensive loss into income	(16.6)
Coal contracts related to forecasted sales
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	29.2	12.1	67.6	115.7
(Loss) gain realized in income on derivatives	12.7	35.1	25.4	97.4
Unrealized gain recognized in income on derivatives		(23.0)	42.2	18.3
Unrealized gain recognized in income on derivatives	16.5
Loss reclassified from other comprehensive loss into income	0.0
Coal trading contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	2.2	(1.6)	(0.3)	(2.9)
(Loss) gain realized in income on derivatives	(11.3)	(8.3)	(8.7)	(5.3)
Unrealized gain recognized in income on derivatives		$ 6.7	$ 8.4	$ 2.4
Unrealized gain recognized in income on derivatives	13.5
Loss reclassified from other comprehensive loss into income	0.0
Commodity swap contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	(11.0)
(Loss) gain realized in income on derivatives	0.0
Unrealized gain recognized in income on derivatives	0.0
Loss reclassified from other comprehensive loss into income	$ (11.0)

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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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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Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

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Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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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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