Derivatives and Fair Value Measurements - Gains and Losses on Hedging Derivatives (Details) - Designated as Hedging Instrument - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Total loss recognized in income	$ (9.6)	$ 48.5
(Loss) gain realized in income on derivatives	(3.3)	4.8
Unrealized gain (loss) recognized in income on derivatives	(6.3)	43.7
Foreign currency option contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total loss recognized in income	(0.9)	(1.1)
(Loss) gain realized in income on derivatives	(1.0)	(1.3)
Unrealized gain (loss) recognized in income on derivatives	0.1	0.2
Coal contracts related to forecasted sales
Derivative Instruments, Gain (Loss) [Line Items]
Total loss recognized in income	(8.6)	50.7
(Loss) gain realized in income on derivatives	(6.4)	10.9
Unrealized gain (loss) recognized in income on derivatives	(2.2)	39.8
Coal trading contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total loss recognized in income	(0.1)	(1.1)
(Loss) gain realized in income on derivatives	4.1	(4.8)
Unrealized gain (loss) recognized in income on derivatives	$ (4.2)	$ 3.7