Derivatives and Fair Value Measurements - Financial Instruments Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 15.4	$ 11.8
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	5.4	1.8
Level 3
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	10.0	10.0
Foreign currency option contracts
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	1.3	1.2
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	1.3	1.2
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	18.8	(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	18.8	(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	(14.7)	21.8
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	(14.7)	21.8
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	10.0	10.0
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	$ 10.0	$ 10.0