Revenue Recognition (Disaggregation of Revenue) (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	$ 382,261	$ 619,467	$ 1,107,014	$ 1,744,872
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	908	954	19,467	4,792
North America revenues
Disaggregation of Revenue [Line Items]
Revenues	256,378	382,248	715,905	1,002,939
North America revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	908	954	19,467	4,792
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	125,883	237,219	391,109	741,933
Seaborne revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
PRB | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	180,850	269,968	492,406	692,845
PRB | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	180,097	269,968	491,471	692,845
PRB | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	753	0	935	0
MET | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	168,054	254,493	489,660	769,000
MET | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	47,197	63,755	117,058	163,317
MET | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	120,857	190,738	372,602	605,683
Other Thermal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	32,449	94,052	105,481	278,235
Other Thermal | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	28,176	47,571	87,909	141,985
Other Thermal | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 4,273	$ 46,481	$ 17,572	$ 136,250