Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred noncurrent income tax assets (liabilities):
Oil & gas properties	$ 11,164	$ 2,998
Basis difference in subsidiary obligation	(2,211)	(2,268)
Investment in Partnerships	(18,517)	(111)
Federal net operating loss carryforward	12,940	12,986
Net deferred noncurrent tax assets	3,376	13,605
Valuation allowance	(16,865)	(24,120)
Net deferred tax liability	$ (13,489)	$ (10,515)