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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	$ 47.7	$ (40.3)	$ 96.4	$ 14.2
(Loss) gain realized in income on derivatives	19.9	10.5	24.8	26.4
Unrealized (loss) gain recognized in income on derivatives	27.8	(50.8)	71.6	(12.2)
Foreign currency option contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	(1.4)	(2.2)	(2.5)	(6.4)
(Loss) gain realized in income on derivatives	(1.1)	(2.3)	(2.4)	(4.7)
Unrealized (loss) gain recognized in income on derivatives	(0.3)	0.1	(0.1)	(1.7)
Coal contracts related to forecasted sales
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	49.4	(36.4)	100.2	23.3
(Loss) gain realized in income on derivatives	27.0	11.7	38.0	32.8
Unrealized (loss) gain recognized in income on derivatives	22.4	(48.1)	62.2	(9.5)
Coal trading contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total (loss) gain recognized in income	(0.3)	(1.7)	(1.3)	(2.7)
(Loss) gain realized in income on derivatives	(6.0)	1.1	(10.8)	(1.7)
Unrealized (loss) gain recognized in income on derivatives	$ 5.7	$ (2.8)	$ 9.5	$ (1.0)