Derivatives and Fair Value Measurements - Derivatives by Balance Sheet Classification (Details) - Designated as Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Asset Derivative
Derivative Asset, Fair Value, Gross Asset	$ 51.4	$ 102.3
Effect of counterparty netting	(32.0)	(74.3)
Variation margin held	(11.1)	(22.1)
Net derivatives and margin as classified in the balance sheets	8.3	5.9
Liability Derivative
Liability Derivative	(32.2)	(74.3)
Effect of counterparty netting	32.0	74.3
Variation margin held	0.0	0.0
Net derivatives and margin as classified in the balance sheets	(0.2)	0.0
Foreign currency option contracts
Asset Derivative
Derivative Asset, Fair Value, Gross Asset	6.1	1.1
Liability Derivative
Liability Derivative	0.0	0.0
Coal contracts related to forecasted sales
Asset Derivative
Derivative Asset, Fair Value, Gross Asset	10.0	20.1
Liability Derivative
Liability Derivative	(1.9)	(0.1)
Coal trading contracts
Asset Derivative
Derivative Asset, Fair Value, Gross Asset	35.3	81.1
Liability Derivative
Liability Derivative	$ (30.3)	$ (74.2)

Amount of obligation to return cash collateral under master netting arrangements offset against derivative assets.

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