Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 319,521	$ 570,222	$ 724,753	$ 1,125,405
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	6,177	623	18,559	3,837
North America revenues
Disaggregation of Revenue [Line Items]
Revenues	210,559	313,553	459,527	620,691
North America revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	6,177	623	18,559	3,837
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	108,962	256,669	265,226	504,714
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	133,096	210,149	311,556	422,878
PRB | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	133,096	210,149	311,375	422,878
PRB | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	0	0	181	0
MET | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	138,951	261,245	321,605	514,507
MET | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	40,137	54,896	69,860	99,562
MET | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	98,814	206,349	251,745	414,945
Other Thermal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	41,297	98,205	73,033	184,183
Other Thermal | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	31,149	47,885	59,733	94,414
Other Thermal | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 10,148	$ 50,320	$ 13,300	$ 89,769