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Derivatives and Fair Value Measurements - Gains and Losses on Hedging Derivatives (Details) - Designated as Hedging Instrument - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]    
Total (loss) gain recognized in income $ 48.5 $ 54.6
(Loss) gain realized in income on derivatives 4.8 16.0
Unrealized (loss) gain recognized in income on derivatives 43.7 38.6
Foreign currency option contracts    
Derivative Instruments, Gain (Loss) [Line Items]    
Total (loss) gain recognized in income (1.1) (4.2)
(Loss) gain realized in income on derivatives (1.3) (2.4)
Unrealized (loss) gain recognized in income on derivatives 0.2 (1.8)
Coal contracts related to forecasted sales    
Derivative Instruments, Gain (Loss) [Line Items]    
Total (loss) gain recognized in income 50.7 59.8
(Loss) gain realized in income on derivatives 10.9 21.2
Unrealized (loss) gain recognized in income on derivatives 39.8 38.6
Coal trading contracts    
Derivative Instruments, Gain (Loss) [Line Items]    
Total (loss) gain recognized in income (1.1) (1.0)
(Loss) gain realized in income on derivatives (4.8) (2.8)
Unrealized (loss) gain recognized in income on derivatives $ 3.7 $ 1.8