Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 357,543	$ 405,232
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	1,222	12,382
North America revenues
Disaggregation of Revenue [Line Items]
Revenues	190,270	248,968
North America revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	1,222	12,382
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	167,273	156,264
Seaborne revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	0	0
PRB | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	178,781	182,654
PRB | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	24,835	29,723
PRB | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	153,946	152,931
MET | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	177,540	210,196
MET | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	164,213	206,863
MET | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	13,327	3,333
Other Thermal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 1,222	$ 12,382