Revenue - Disaggregation of Revenue by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 410,614	$ 653,828	$ 878,981	$ 1,260,788
Coal
Disaggregation of Revenue [Line Items]
Revenues	410,614	653,828	878,981	1,260,788
Met
Disaggregation of Revenue [Line Items]
Revenues	324,808	501,629	688,851	972,073
Thermal
Disaggregation of Revenue [Line Items]
Revenues	85,806	152,199	190,130	288,715
Export coal revenues
Disaggregation of Revenue [Line Items]
Revenues	252,273	362,838	509,011	706,144
Export coal revenues | Coal
Disaggregation of Revenue [Line Items]
Revenues	252,274	362,838	509,012	706,144
Export coal revenues | Met
Disaggregation of Revenue [Line Items]
Revenues	241,905	345,576	491,260	679,762
Export coal revenues | Thermal
Disaggregation of Revenue [Line Items]
Revenues	10,369	17,262	17,752	26,382
Domestic coal revenues | Coal
Disaggregation of Revenue [Line Items]
Revenues	158,340	290,990	369,969	554,644
Domestic coal revenues | Met
Disaggregation of Revenue [Line Items]
Revenues	82,903	156,053	197,591	292,311
Domestic coal revenues | Thermal
Disaggregation of Revenue [Line Items]
Revenues	$ 75,437	$ 134,937	$ 172,378	$ 262,333