Supplemental Disclosures to the Financial Statements - Accounts Payable and Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accounts payable-trade	$ 22,498	$ 11,055
Accrued expenses	43,849	43,452
Royalties payable	16,921	15,150
Greenhouse gas liability - current portion	34,123	35,554
Taxes other than income tax liability	9,892	10,118
Accrued interest	3,500	10,783
Dividends payable	3,218	0
Asset retirement obligations - current portion	25,000	25,000
Other	845	873
Total accounts payable and accrued expenses	$ 159,846	$ 151,985