Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 555,183	$ 575,295
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	3,214	0
North America revenues
Disaggregation of Revenue [Line Items]
Revenues	307,138	317,705
North America revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	3,214	0
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	248,045	257,590
Seaborne revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	0	0
PRB | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	212,729	245,428
PRB | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	212,729	244,360
PRB | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	0	1,068
MET | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	253,262	238,347
MET | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	44,666	29,678
MET | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	208,596	208,669
Other Thermal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	85,978	91,520
Other Thermal | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	46,529	43,667
Other Thermal | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 39,449	$ 47,853