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Derivatives and Fair Value Measurements - Textual (Details)
T in Millions, $ in Millions
Dec. 31, 2021
T
Sep. 30, 2021
$ / $
Dec. 31, 2020
AUD ($)
T
Dec. 31, 2020
USD ($)
T
Dec. 31, 2019
USD ($)
Coal Trading          
Derivatives, Fair Value [Line Items]          
Derivative, Collateral, Right to Reclaim Cash       $ 3.0 $ 7.9
Receivables from Brokers-Dealers and Clearing Organizations       0.9 1.3
Derivative, Fair Value, Amount Offset Against Collateral, Net       6.5 22.1
Potential collateralization that may be requested by counterparties due to a material adverse event       $ 1.6 $ 0.1
Foreign currency option contracts          
Derivatives, Fair Value [Line Items]          
Derivative, Notional Amount     $ 575,000,000.0    
Coal          
Derivatives, Fair Value [Line Items]          
Derivative, Nonmonetary Notional Amount | T     0.8 0.8  
Diesel Fuel Hedge Contracts          
Derivatives, Fair Value [Line Items]          
Interest Rate Derivatives, at Fair Value, Net     $ 0    
Scenario, Forecast | Foreign currency option contracts | Minimum          
Derivatives, Fair Value [Line Items]          
Derivative, Exchange Rate Cap | $ / $   0.75      
Scenario, Forecast | Foreign currency option contracts | Maximum          
Derivatives, Fair Value [Line Items]          
Derivative, Exchange Rate Cap | $ / $   0.81      
Scenario, Forecast | Coal          
Derivatives, Fair Value [Line Items]          
Derivative, Nonmonetary Notional Amount | T 0.8