Derivatives and Fair Value Measurements - Derivatives by Balance Sheet Classification (Details) - Designated as Hedging Instrument - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Asset Derivative
Total derivatives	$ 223.1	$ 67.5
Effect of counterparty netting	(171.8)	(64.5)
Variation margin (held) posted	(46.1)	0.0
Net derivatives and margin as classified in the balance sheets	5.2	3.0
Liability Derivative
Total derivatives	(171.8)	(87.5)
Effect of counterparty netting	171.8	64.5
Variation margin (held) posted	0.0	21.8
Net derivatives and margin as classified in the balance sheets	0.0	(1.2)
Foreign currency option contracts
Asset Derivative
Foreign currency option contracts	1.2	1.2
Liability Derivative
Foreign currency option contracts	0.0	0.0
Coal contracts related to forecasted sales
Asset Derivative
Coal contracts related to forecasted sales	34.7	6.6
Liability Derivative
Coal contracts related to forecasted sales	(3.7)	(23.1)
Coal trading contracts
Asset Derivative
Coal trading contracts	187.2	59.7
Liability Derivative
Coal trading contracts	$ (168.1)	$ (64.4)

Coal contracts related to forecasted sales, asset position

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