Revenue from Contracts with Customers - Schedule of Partnerships' Coal Royalty and Other Segment Revenues from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	$ 49,371					$ 152,150
Revenues	94,855	$ 47,920	$ 44,474	$ 93,116	$ 92,394	291,345	$ 273,339
Coal Royalty and Other
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	42,518					134,912
Revenues	42,459			43,507		134,428	111,269
Coal Royalty and Other | Coal royalty revenue
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	30,709					96,473
Coal Royalty and Other | Production lease minimum revenue
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	1,769					6,310
Coal Royalty and Other | Minimum lease straight-line revenue
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	567					1,739
Coal Royalty and Other | Wheelage revenue
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	1,572					5,155
Coal Royalty and Other | Coal overriding royalty revenue
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	3,918					10,492
Coal Royalty and Other | Aggregates royalty revenue
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	888					3,551
Coal Royalty and Other | Oil and gas royalty revenue
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	1,427					5,679
Coal Royalty and Other | Property tax revenue
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	1,263					3,968
Coal Royalty and Other | Other revenue
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	405					1,545
Coal Royalty and Other | Transportation and processing services revenue
Disaggregation of Revenue [Line Items]
Total Coal royalty and other segment revenues	3,600					9,600
Revenues	6,853			$ 5,571		17,238	$ 9,717
Coal Royalty and Other | Transportation and Processing
Disaggregation of Revenue [Line Items]
Lease revenue	$ 3,300					$ 7,600