Supplementary Information on Oil and Gas Operations—Unaudited - Standardized Measure (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Net Cash Flows [Abstract]
Future cash flows	$ 539,244	$ 299,035	$ 487,834
Future production costs	(184,171)	(117,283)	(171,678)
Future development costs	(128,447)	(83,720)	(116,591)
Future income taxes	0	0	0
Future net cash flows	226,626	98,032	199,565
10% annual discount	(99,329)	(30,763)	(71,880)
Standardized measure of discounted future net cash flows	$ 127,297	$ 67,269	$ 127,685	$ 548,562