Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred noncurrent income tax assets (liabilities):
Oil & gas properties	$ 20,633	$ 11,164
Basis difference in subsidiary obligation	(2,211)	(2,211)
Investment in Partnerships	(31,722)	(18,517)
Federal net operating loss carryforward	14,597	12,940
Net deferred noncurrent tax assets	1,297	3,376
Valuation allowance	(16,451)	(16,865)
Net deferred tax liability	$ (15,154)	$ (13,489)