XML 88 R63.htm IDEA: XBRL DOCUMENT v3.20.4
Joint Venture with Peabody Energy (Details) - Implementation Agreement - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Jun. 18, 2019
Schedule of Equity Method Investments [Line Items]      
Economic interest held     33.50%
Incurred expenses related to regulatory approval process $ 16.1 $ 13.8  
Peabody Energy Corporation      
Schedule of Equity Method Investments [Line Items]      
Economic interest held     66.50%