Summary of Significant Accounting Policies - Revenue Recognition (Details) $ in Thousands	3 Months Ended	14 Months Ended
Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	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	$ 226,720	$ 378,290
XCoal \| Customer Concentration Risk \| Revenue
Concentration Risk [Line Items]
Concentration risk, percentage	19.90%	25.80%
Other revenues	$ 44,300	$ 94,900

Strategic Partner Arrangement, Maximum Total Production Transferred For Title And Marketing

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