Derivatives and Fair Value Measurements - Financial Instruments Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Net financial asset (liability)	$ 13.2	$ 9.9
Level 1
Derivative [Line Items]
Net financial asset (liability)	0.0	0.0
Level 2
Derivative [Line Items]
Net financial asset (liability)	9.2	5.9
Level 3
Derivative [Line Items]
Net financial asset (liability)	4.0	4.0
Foreign currency option contracts
Derivative [Line Items]
Net financial asset (liability)	10.3	1.1
Foreign currency option contracts | Level 1
Derivative [Line Items]
Net financial asset (liability)	0.0	0.0
Foreign currency option contracts | Level 2
Derivative [Line Items]
Net financial asset (liability)	10.3	1.1
Foreign currency option contracts | Level 3
Derivative [Line Items]
Net financial asset (liability)	0.0	0.0
Coal contracts related to forecasted sales
Derivative [Line Items]
Net financial asset (liability)	(7.9)	21.2
Coal contracts related to forecasted sales | Level 1
Derivative [Line Items]
Net financial asset (liability)	0.0	0.0
Coal contracts related to forecasted sales | Level 2
Derivative [Line Items]
Net financial asset (liability)	(7.9)	21.2
Coal contracts related to forecasted sales | Level 3
Derivative [Line Items]
Net financial asset (liability)	0.0	0.0
Coal trading contracts
Derivative [Line Items]
Net financial asset (liability)	6.8	(16.4)
Coal trading contracts | Level 1
Derivative [Line Items]
Net financial asset (liability)	0.0	0.0
Coal trading contracts | Level 2
Derivative [Line Items]
Net financial asset (liability)	6.8	(16.4)
Coal trading contracts | Level 3
Derivative [Line Items]
Net financial asset (liability)	0.0	0.0
Equity Securities
Derivative [Line Items]
Net financial asset (liability)	4.0	4.0
Equity Securities | Level 1
Derivative [Line Items]
Net financial asset (liability)	0.0	0.0
Equity Securities | Level 2
Derivative [Line Items]
Net financial asset (liability)	0.0	0.0
Equity Securities | Level 3
Derivative [Line Items]
Net financial asset (liability)	$ 4.0	$ 4.0