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Summary of Significant Accounting Policies - Revenue Recognition (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) t	Jun. 30, 2020 USD ($)
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	$ 227,440	$ 163,701	$ 441,204	$ 390,421
XCoal | Customer Concentration Risk | Revenue
Concentration Risk [Line Items]
Other revenues	$ 141,800	$ 0	$ 248,100	$ 43,600
Concentration risk, percentage	63.10%		57.30%	11.40%

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