Revenue Recognition (Schedule of Revenues) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 1,467,592	$ 2,294,352	$ 2,451,787
North America revenues
Disaggregation of Revenue [Line Items]
Revenues	970,662	1,334,032	1,340,887
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	496,930	960,320	1,110,900
Operating Segments | PRB
Disaggregation of Revenue [Line Items]
Revenues	662,135	915,750	973,248
Operating Segments | PRB | North America revenues
Disaggregation of Revenue [Line Items]
Revenues	659,807	915,750	971,337
Operating Segments | PRB | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	2,328	0	1,911
Operating Segments | MET
Disaggregation of Revenue [Line Items]
Revenues	641,536	990,550	1,036,621
Operating Segments | MET | North America revenues
Disaggregation of Revenue [Line Items]
Revenues	173,508	217,381	160,969
Operating Segments | MET | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	468,028	773,169	875,652
Operating Segments | Other Thermal
Disaggregation of Revenue [Line Items]
Revenues	139,497	377,202	428,884
Operating Segments | Other Thermal | North America revenues
Disaggregation of Revenue [Line Items]
Revenues	112,923	190,051	195,547
Operating Segments | Other Thermal | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	26,574	187,151	233,337
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	24,424	10,850	13,034
Corporate, Other and Eliminations | North America revenues
Disaggregation of Revenue [Line Items]
Revenues	24,424	10,850	13,034
Corporate, Other and Eliminations | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0	$ 0