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Segment Information - Revenue from External Customers (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]    
Coal revenues $ 468,367 $ 606,960
Export coal revenues    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Coal revenues 256,738 343,306
India    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Coal revenues 55,985 $ 125,598
Brazil    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Coal revenues $ 50,580  
Geographic Concentration Risk | Export coal revenues | Revenues    
Revenues from External Customers and Long-Lived Assets [Line Items]    
Concentration risk percentage 55.00% 57.00%