Schedule of Results of Operations from Oil, Natural Gas and Natural Gas Liquids Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Results Of Operations Oil And Gas Producing Activities Net Income Excluding Corporate Overhead And Interest Costs [Abstract]
Royalty income	$ 40,081	$ 68,463	$ 98,655
Production and ad valorem taxes	(2,807)	(4,262)	(5,143)
Marketing and transportation	(1,993)	(2,396)	(2,368)
Depletion	(14,103)	(12,737)	(16,962)
Income tax expense	(589)	(3,918)	(3,292)
Results of operations from oil, natural gas and natural gas liquids	$ 20,589	$ 45,150	$ 70,890