Summary of Significant Accounting Policies - Revenue Recognition (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) t	Sep. 30, 2019 USD ($)
Concentration Risk [Line Items]
Percent of total production transferred for title and marketing			10.00%
Maximum total production transferred for title and marketing (in tons) | t			250,000
Other revenues	$ 180,064	$ 287,506	$ 570,484	$ 1,063,408
XCoal | Customer Concentration Risk | Revenue
Concentration Risk [Line Items]
Other revenues	$ 38,700	$ 57,800	$ 82,300	$ 220,800
Concentration risk, percentage	22.10%	20.60%	14.80%	21.20%