Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 633,180	$ 613,538	$ 1,800,824	$ 1,764,379
Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	4,262	4,733	7,887	4,790
North America revenues
Disaggregation of Revenue [Line Items]
Revenues	371,938	441,570	1,001,173	1,237,370
North America revenues | Corporate, Other and Eliminations
Disaggregation of Revenue [Line Items]
Revenues	4,262	4,733	7,887	4,790
Seaborne revenues
Disaggregation of Revenue [Line Items]
Revenues	261,242	171,968	799,651	527,009
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	261,927	276,000	737,233	780,007
PRB | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	261,927	276,000	735,322	780,007
PRB | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	0	0	1,911	0
MET | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	236,328	238,946	733,707	692,178
MET | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	49,698	84,411	117,699	235,644
MET | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	186,630	154,535	616,008	456,534
Other Thermal | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	130,663	93,859	321,997	287,404
Other Thermal | North America revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	56,051	76,426	140,265	216,929
Other Thermal | Seaborne revenues | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 74,612	$ 17,433	$ 181,732	$ 70,475