Summary of Significant Accounting Policies - Revenue Recognition (Details)	3 Months Ended		6 Months Ended		14 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2018 t
Concentration Risk [Line Items]
Percent of total production transferred for title and marketing					10.00%
Maximum total production transferred for title and marketing | t					250,000
Other revenues	$ 163,701,000	$ 397,613,000	$ 390,421,000	$ 775,903,000
XCoal | Customer Concentration Risk | Revenue
Concentration Risk [Line Items]
Other revenues	$ 0	$ 68,100,000	$ 43,600,000	$ 163,000,000.0
Concentration risk, percentage		17.50%	11.40%	21.70%