Derivatives and Fair Value Measurements - Financial Instruments Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 7.7	$ 9.9
Level 1
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0.0	0.0
Level 2
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	3.7	5.9
Level 3
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	4.0	4.0
Foreign currency option contracts
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0.2	1.1
Foreign currency option contracts | Level 1
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0.0	0.0
Foreign currency option contracts | Level 2
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0.2	1.1
Foreign currency option contracts | Level 3
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0.0	0.0
Coal contracts related to forecasted sales
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	20.0	21.2
Coal contracts related to forecasted sales | Level 1
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0.0	0.0
Coal contracts related to forecasted sales | Level 2
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	20.0	21.2
Coal contracts related to forecasted sales | Level 3
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0.0	0.0
Coal trading contracts
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	(16.5)	(16.4)
Coal trading contracts | Level 1
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0.0	0.0
Coal trading contracts | Level 2
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	(16.5)	(16.4)
Coal trading contracts | Level 3
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0.0	0.0
Equity securities
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	4.0	4.0
Equity securities | Level 1
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0.0	0.0
Equity securities | Level 2
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0.0	0.0
Equity securities | Level 3
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 4.0	$ 4.0