Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 619,467	$ 633,180	$ 1,744,872	$ 1,800,824
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	955	4,262	4,792	7,887
North America revenues
Disaggregation of Revenue [Line Items]
Revenues	382,248	371,938	1,002,939	1,001,173
North America revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	955	4,262	4,792	7,887
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	237,219	261,242	741,933	799,651
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	269,967	261,927	692,845	737,233
PRB | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	269,967	261,927	692,845	735,322
PRB | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	1,911
MET | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	254,493	236,328	769,000	733,707
MET | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	63,755	49,698	163,317	117,699
MET | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	190,738	186,630	605,683	616,008
Other Thermal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	94,052	130,663	278,235	321,997
Other Thermal | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	47,571	56,051	141,985	140,265
Other Thermal | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 46,481	$ 74,612	$ 136,250	$ 181,732