Derivatives and Fair Value Measurements - Gains and Losses on Hedging Derivatives (Details) - Designated as Hedging Instrument - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	$ (7.4)	$ 14.7	$ (2.9)	$ 5.1
Gain (loss) realized in income on derivatives	13.7	3.2	30.7	0.0
Unrealized (loss) gain recognized in income on derivatives	(21.1)	11.5	(33.6)	5.1
Foreign currency option contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	(1.2)	2.2	(4.2)	1.3
Gain (loss) realized in income on derivatives	0.0	(0.6)	4.6	(1.6)
Unrealized (loss) gain recognized in income on derivatives	(1.2)	2.8	(8.8)	2.9
Coal contracts related to forecasted sales
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	(7.6)	12.6	0.6	4.1
Gain (loss) realized in income on derivatives	16.1	5.7	26.2	(0.6)
Unrealized (loss) gain recognized in income on derivatives	(23.7)	6.9	(25.6)	4.7
Coal trading contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	1.4	(0.1)	0.7	(0.3)
Gain (loss) realized in income on derivatives	(2.4)	(1.9)	(0.1)	2.2
Unrealized (loss) gain recognized in income on derivatives	$ 3.8	$ 1.8	$ 0.8	$ (2.5)