Derivatives and Fair Value Measurements - Derivatives by Balance Sheet Classification (Details) - Designated as Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Asset Derivative
Asset derivatives, gross	$ 169.2	$ 67.5
Effect of counterparty netting, derivative assets	(145.5)	(64.5)
Variation margin posted on derivative assets	(18.2)	0.0
Net derivative assets and margin as classified in the balance sheets	5.5	3.0
Liability Derivative
Liability derivatives, gross	(145.6)	(87.5)
Effect of counterparty netting, derivative liabilities	145.5	64.5
Variation margin posted on derivative liabilities	0.0	21.8
Net derivative liabilities and margin as classified in the balance sheets	(0.1)	(1.2)
Foreign currency option contracts
Asset Derivative
Asset derivatives, gross	1.3	1.2
Liability Derivative
Liability derivatives, gross	0.0	0.0
Coal contracts related to forecasted sales
Asset Derivative
Asset derivatives, gross	23.0	6.6
Liability Derivative
Liability derivatives, gross	(7.9)	(23.1)
Coal trading contracts
Asset Derivative
Asset derivatives, gross	144.9	59.7
Liability Derivative
Liability derivatives, gross	$ (137.7)	$ (64.4)