Supplementary Information on Oil and Gas Operations—Unaudited - Changes in Standardized Measure (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Roll Forward]
Standardized measure at beginning of year	$ 127,297	$ 67,269	$ 127,685
Sales and transfers of oil and gas produced, net of production costs	(64,148)	(70,362)	(35,993)
Changes in price, net of future production costs	18,542	53,516	(30,427)
Extensions and discoveries, net of future production and development costs	2,983	50,977	864
Changes in estimated future development costs, net of development costs incurred during this period	16,241	17,144	26,356
Revisions of quantity estimates	2,755	(7,482)	(14,889)
Accretion of discount	12,730	6,727	12,769
Purchase of reserves in place	0	549	0
Sale of reserves in place	1,614	(1,305)	(16,701)
Changes in production rates (timing) and other	6,011	10,264	(2,395)
Net increase (decrease) in standardized measure	(3,272)	60,028	(60,416)
Standardized measure at end of year	$ 124,025	$ 127,297	$ 67,269