Revenue - Disaggregation of Revenue by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 399,954	$ 523,987	$ 1,278,935	$ 1,784,775
Coal
Disaggregation of Revenue [Line Items]
Revenues	399,954	523,987	1,278,935	1,784,775
Met
Disaggregation of Revenue [Line Items]
Revenues	287,014	385,774	975,865	1,357,847
Thermal
Disaggregation of Revenue [Line Items]
Revenues	112,940	138,213	303,070	426,928
Export coal revenues
Disaggregation of Revenue [Line Items]
Revenues	209,046	261,815	718,058	967,959
Export coal revenues | Coal
Disaggregation of Revenue [Line Items]
Revenues	209,046	261,815	718,058	967,959
Export coal revenues | Met
Disaggregation of Revenue [Line Items]
Revenues	202,339	248,628	693,599	928,390
Export coal revenues | Thermal
Disaggregation of Revenue [Line Items]
Revenues	6,707	13,187	24,459	39,569
Domestic coal revenues | Coal
Disaggregation of Revenue [Line Items]
Revenues	190,908	262,172	560,877	816,816
Domestic coal revenues | Met
Disaggregation of Revenue [Line Items]
Revenues	84,675	137,146	282,266	429,457
Domestic coal revenues | Thermal
Disaggregation of Revenue [Line Items]
Revenues	$ 106,233	$ 125,026	$ 278,611	$ 387,359