Derivatives and Fair Value Measurements - Derivatives by Balance Sheet Classification (Details) - Designated as Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Asset Derivative
Derivative Asset, Fair Value, Gross Asset	$ 34.2	$ 34.6
Effect of counterparty netting	(44.1)	(30.2)
Variation margin posted	13.6	6.5
Net derivatives and margin as classified in the balance sheets	3.7	10.9
Liability Derivative
Liability Derivative	(44.1)	(31.9)
Effect of counterparty netting	44.1	30.2
Variation margin posted	0.0	0.0
Net derivatives and margin as classified in the balance sheets	0.0	(1.7)
Foreign currency option contracts
Asset Derivative
Derivative Asset, Fair Value, Gross Asset	2.7	10.3
Liability Derivative
Liability Derivative	0.0	0.0
Coal contracts related to forecasted sales
Asset Derivative
Derivative Asset, Fair Value, Gross Asset	1.1	0.9
Liability Derivative
Liability Derivative	(8.6)	(8.8)
Coal trading contracts
Asset Derivative
Derivative Asset, Fair Value, Gross Asset	30.4	23.4
Liability Derivative
Liability Derivative	$ (35.5)	$ (23.1)