Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 570,222	$ 592,349	$ 1,125,405	$ 1,167,644
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	623	3,625	3,837	3,625
North America revenues
Disaggregation of Revenue [Line Items]
Revenues	313,553	311,530	620,691	629,235
North America revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	623	3,625	3,837	3,625
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	256,669	280,819	504,714	538,409
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	210,149	229,878	422,878	475,306
PRB | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	210,149	229,035	422,878	473,395
PRB | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	0	843	0	1,911
MET | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	261,245	259,032	514,507	497,379
MET | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	54,896	38,323	99,562	68,001
MET | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	206,349	220,709	414,945	429,378
Other Thermal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	98,205	99,814	184,183	191,334
Other Thermal | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	47,885	40,547	94,414	84,214
Other Thermal | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 50,320	$ 59,267	$ 89,769	$ 107,120