Supplementary Information on Oil and Gas Operations—Unaudited - Changes in Standardized Measure (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Standardized measure at beginning of year	$ 67,269	$ 127,685	$ 548,562
Sales and transfers of oil and gas produced, net of production costs	(70,362)	(35,993)	(55,849)
Changes in price, net of future production costs	53,516	(30,427)	(267,710)
Extensions and discoveries, net of future production and development costs	50,977	864	70,928
Changes in estimated future development costs, net of development costs incurred during this period	17,144	26,356	31,007
Revisions of quantity estimates	(7,482)	(14,889)	(14,427)
Accretion of discount	6,727	12,769	60,071
Net change in income taxes	0	0	52,149
Purchase of reserves in place	549	0	0
Sale of reserves in place	(1,305)	(16,701)	(194,454)
Changes in production rates (timing) and other	10,264	(2,395)	(102,592)
Net increase (decrease) in standardized measure	60,028	(60,416)	(420,877)
Standardized measure at end of year	$ 127,297	$ 67,269	$ 127,685