Supplemental Disclosures to the Financial Statements - Accounts Payable and Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accounts payable-trade	$ 6,596	$ 13,986
Accrued expenses	45,841	57,078
Royalties payable	12,282	25,385
Taxes other than income tax liability	12,390	9,150
Accrued interest	3,500	10,500
Dividends payable	5	9,888
Asset retirement obligation - current portion	13,700	25,208
Other	923	616
Total accounts payable and accrued expenses	$ 95,237	$ 151,811