Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 592,349	$ 549,866	$ 1,167,644	$ 1,150,841
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	3,625	(1)	3,625	57
North America revenues
Disaggregation of Revenue [Line Items]
Revenues	311,530	384,615	629,235	795,800
North America revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	3,625	(1)	3,625	57
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	280,819	165,251	538,409	355,041
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	229,878	230,579	475,306	504,007
PRB | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	229,035	230,579	473,395	504,007
PRB | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	843	0	1,911	0
MET | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	259,032	227,649	497,379	453,232
MET | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	38,323	82,498	68,001	151,233
MET | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	220,709	145,151	429,378	301,999
Other Thermal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	99,814	91,639	191,334	193,545
Other Thermal | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	40,547	71,539	84,214	140,503
Other Thermal | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 59,267	$ 20,100	$ 107,120	$ 53,042