Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred noncurrent income tax assets (liabilities):
Oil & gas properties	$ 18,929	$ 20,633
Basis difference in subsidiary obligation	(2,211)	(2,211)
Investment in Partnerships	(25,760)	(31,722)
Federal net operating loss carryforward	11,590	14,597
Net deferred noncurrent tax assets	2,548	1,297
Valuation allowance	(17,044)	(16,451)
Net deferred tax liability	$ (14,496)	$ (15,154)