Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 405,232	$ 555,183
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	12,382	3,214
North America revenues
Disaggregation of Revenue [Line Items]
Revenues	248,968	307,138
North America revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	12,382	3,214
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	156,264	248,045
Seaborne revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	0	0
PRB | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	178,460	212,729
PRB | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	178,279	212,729
PRB | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	181	0
MET | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	182,654	253,262
MET | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	29,723	44,666
MET | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	152,931	208,596
Other Thermal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	31,736	85,978
Other Thermal | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	28,584	46,529
Other Thermal | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 3,152	$ 39,449