Revenue from Contracts with Customers - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue from External Customer [Line Items]
Accounts Receivable Net, Including Related Party Receivables	$ 32,058	$ 29,086
Current portion of deferred revenue	3,509		$ 0
Deferred revenue	49,044		$ 100,605
Coal Royalty
Revenue from External Customer [Line Items]
Accounts Receivable Net, Including Related Party Receivables	29,001	25,443
Prepaid Expense and Other Assets	2,483	2,830
Current portion of deferred revenue	3,509	1,022
Deferred revenue	49,044	33,992
Accounting Standards Update 2014-09
Revenue from External Customer [Line Items]
Accrued Fees and Other Revenue Receivable	1,900	$ 1,900
Accounting Standards Update 2014-09 | Coal Royalty
Revenue from External Customer [Line Items]
Accrued Fees and Other Revenue Receivable	$ 4,430