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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	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income	$ (11.6)	$ 63.6	$ 103.7
Gain realized in income on derivatives	15.8	11.8	83.7
Unrealized gain (loss) recognized in income on derivatives	(27.4)	51.8	20.0
Foreign currency option contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income	12.9	(3.7)	(9.1)
Gain realized in income on derivatives	5.8	(4.9)	(8.4)
Unrealized gain (loss) recognized in income on derivatives	7.1	1.2	(0.7)
Coal contracts related to forecasted sales
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income	(23.8)	67.6	115.7
Gain realized in income on derivatives	5.8	25.4	97.4
Unrealized gain (loss) recognized in income on derivatives	(29.6)	42.2	18.3
Coal trading contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized in income	(0.7)	(0.3)	(2.9)
Gain realized in income on derivatives	4.2	(8.7)	(5.3)
Unrealized gain (loss) recognized in income on derivatives	$ (4.9)	$ 8.4	$ 2.4

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- Definition

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

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Amount, before tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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