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Derivatives and Fair Value Measurements - Derivatives by Balance Sheet Classification (Details) - Designated as Hedging Instrument - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Asset Derivative    
Derivative Asset, Fair Value, Gross Asset $ 86.0 $ 34.6
Effect of counterparty netting (116.0) (30.2)
Variation margin posted 31.9 6.5
Net derivatives and margin as classified in the balance sheets 1.9 10.9
Liability Derivative    
Liability Derivative (117.5) (31.9)
Effect of counterparty netting 116.0 30.2
Variation margin posted 0.0 0.0
Net derivatives and margin as classified in the balance sheets (1.5) (1.7)
Foreign currency option contracts    
Asset Derivative    
Derivative Asset, Fair Value, Gross Asset 1.0 10.3
Liability Derivative    
Liability Derivative 0.0 0.0
Coal contracts related to forecasted sales    
Asset Derivative    
Derivative Asset, Fair Value, Gross Asset 1.8 0.9
Liability Derivative    
Liability Derivative (28.9) (8.8)
Coal trading contracts    
Asset Derivative    
Derivative Asset, Fair Value, Gross Asset 83.2 23.4
Liability Derivative    
Liability Derivative $ (88.6) $ (23.1)