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Summary of Significant Accounting Policies - Revenue Recognition (Details)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2021
USD ($)
t
Sep. 30, 2020
USD ($)
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 $ 202,467 $ 180,064 $ 643,671 $ 570,485
XCoal | Customer Concentration Risk | Revenue        
Concentration Risk [Line Items]        
Other revenues $ 122,300 $ 38,700 $ 370,400 $ 82,300
Concentration risk, percentage 61.10% 22.10% 58.50% 14.80%