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%