Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 575,295	$ 600,975
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Total revenues	0	59
North America revenues
Disaggregation of Revenue [Line Items]
Domestic and export revenues	317,705	411,185
North America revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Domestic and export revenues	0	59
Seaborne revenues
Disaggregation of Revenue [Line Items]
Domestic and export revenues	257,590	189,790
Seaborne revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Domestic and export revenues	0	0
PRB
Disaggregation of Revenue [Line Items]
Total revenues	245,428	273,428
PRB | North America revenues
Disaggregation of Revenue [Line Items]
Domestic and export revenues	244,360	273,428
PRB | Seaborne revenues
Disaggregation of Revenue [Line Items]
Domestic and export revenues	1,068	0
MET
Disaggregation of Revenue [Line Items]
Total revenues	238,347	225,582
MET | North America revenues
Disaggregation of Revenue [Line Items]
Domestic and export revenues	29,678	68,734
MET | Seaborne revenues
Disaggregation of Revenue [Line Items]
Domestic and export revenues	208,669	156,848
Other Thermal
Disaggregation of Revenue [Line Items]
Total revenues	91,520	101,906
Other Thermal | North America revenues
Disaggregation of Revenue [Line Items]
Domestic and export revenues	43,667	68,964
Other Thermal | Seaborne revenues
Disaggregation of Revenue [Line Items]
Domestic and export revenues	$ 47,853	$ 32,942