Revenue - Disaggregation of Revenue by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 468,367	$ 606,960
Coal
Disaggregation of Revenue [Line Items]
Revenues	468,367	606,960
Met
Disaggregation of Revenue [Line Items]
Revenues	364,043	470,443
Thermal
Disaggregation of Revenue [Line Items]
Revenues	104,324	136,517
Export coal revenues
Disaggregation of Revenue [Line Items]
Revenues	256,738	343,306
Export coal revenues | Coal
Disaggregation of Revenue [Line Items]
Revenues	256,738	343,306
Export coal revenues | Met
Disaggregation of Revenue [Line Items]
Revenues	249,355	334,185
Export coal revenues | Thermal
Disaggregation of Revenue [Line Items]
Revenues	7,383	9,121
Domestic coal revenues | Coal
Disaggregation of Revenue [Line Items]
Revenues	211,629	263,654
Domestic coal revenues | Met
Disaggregation of Revenue [Line Items]
Revenues	114,688	136,258
Domestic coal revenues | Thermal
Disaggregation of Revenue [Line Items]
Revenues	$ 96,941	$ 127,396