XML 64 R48.htm IDEA: XBRL DOCUMENT v3.20.2
Joint Venture with Peabody Energy (Details) - Implementation Agreement - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Jun. 18, 2019
Schedule of Equity Method Investments [Line Items]          
Economic interest held         33.50%
Incurred expenses related to regulatory approval process $ 4.4 $ 3.8 $ 15.9 $ 6.8  
Peabody Energy Corporation          
Schedule of Equity Method Investments [Line Items]          
Economic interest held         66.50%