Revenue Recognition (Schedule of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 549,480	$ 619,467	$ 570,222	$ 555,183	$ 650,963	$ 633,180	$ 592,349	$ 575,295	$ 2,294,352	$ 2,451,787	$ 2,324,623
North America revenues
Disaggregation of Revenue [Line Items]
Revenues									1,334,032	1,340,887	1,622,161
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues									960,320	1,110,900	702,462
Operating segments | PRB
Disaggregation of Revenue [Line Items]
Revenues									915,750	973,248	1,024,197
Operating segments | PRB | North America revenues
Disaggregation of Revenue [Line Items]
Revenues									915,750	971,337	1,024,197
Operating segments | PRB | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues									0	1,911	0
Operating segments | MET
Disaggregation of Revenue [Line Items]
Revenues									990,550	1,036,621	887,839
Operating segments | MET | North America revenues
Disaggregation of Revenue [Line Items]
Revenues									217,381	160,969	312,874
Operating segments | MET | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues									773,169	875,652	574,965
Operating segments | Other Thermal
Disaggregation of Revenue [Line Items]
Revenues									377,202	428,884	396,504
Operating segments | Other Thermal | North America revenues
Disaggregation of Revenue [Line Items]
Revenues									190,051	195,547	269,007
Operating segments | Other Thermal | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues									187,151	233,337	127,497
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues									(10,850)	(13,034)	(16,083)
Corporate, Other and Eliminations | North America revenues
Disaggregation of Revenue [Line Items]
Revenues									(10,850)	(13,034)	(16,083)
Corporate, Other and Eliminations | Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues									$ 0	$ 0	$ 0