Derivatives and Fair Value Measurements - Gains and Losses on Hedging Derivatives (Details) - Designated as Hedging Instrument - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	$ 4.6	$ (9.6)
Gain realized in income on derivatives	17.1	(3.3)
Unrealized loss recognized in income on derivatives	(12.5)	(6.3)
Foreign currency option contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	(2.9)	(0.9)
Gain realized in income on derivatives	4.7	(1.0)
Unrealized loss recognized in income on derivatives	(7.6)	0.1
Coal contracts related to forecasted sales
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	8.2	(8.6)
Gain realized in income on derivatives	10.0	(6.4)
Unrealized loss recognized in income on derivatives	(1.8)	(2.2)
Coal trading contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	(0.7)	(0.1)
Gain realized in income on derivatives	2.4	4.1
Unrealized loss recognized in income on derivatives	$ (3.1)	$ (4.2)