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Joint Venture with Peabody Energy (Details) - Implementation Agreement - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2020
Jun. 30, 2019
Jun. 18, 2019
Schedule of Equity Method Investments [Line Items]        
Economic interest held       33.50%
Incurred expenses related to regulatory approval process $ 7.9 $ 11.5 $ 3.0  
Peabody Energy Corporation        
Schedule of Equity Method Investments [Line Items]        
Economic interest held       66.50%