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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	$ 43,784	$ 49,371	$ 168,777	$ 152,150
Revenues	57,602	58,207	205,610	187,136
Coal Royalty and Other
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	39,919	42,518	154,037	134,912
Revenues	39,919	42,518	154,037	134,912
Coal Royalty and Other | Coal royalty revenues
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	24,727	30,709	87,561	96,473
Coal Royalty and Other | Production lease minimum revenues
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	2,752	1,769	21,331	6,310
Coal Royalty and Other | Minimum lease straight-line revenues
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	3,982	567	11,152	1,739
Coal Royalty and Other | Property tax revenues
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	1,606	1,263	4,416	3,968
Coal Royalty and Other | Wheelage revenues
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	1,675	1,572	5,035	5,155
Coal Royalty and Other | Coal overriding royalty revenues
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	2,189	3,918	10,163	10,492
Coal Royalty and Other | Lease amendment revenues
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	1,535	0	6,720	0
Coal Royalty and Other | Aggregates royalty revenues
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	954	888	3,655	3,551
Coal Royalty and Other | Oil and gas royalty revenues
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	374	1,427	2,575	5,679
Coal Royalty and Other | Other revenues
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	125	405	1,429	1,545
Coal Royalty and Other | Transportation and processing services revenues
Disaggregation of Revenue [Line Items]
Total Coal Royalty and Other segment revenues	1,700	3,600	7,300	9,600
Revenues	3,865	6,853	14,740	17,238
Coal Royalty and Other | Transportation and processing revenues
Disaggregation of Revenue [Line Items]
Lease revenue	$ 2,200	$ 3,300	$ 7,500	$ 7,600