Oil and Gas Producing Activities (Unaudited) - Effect of income taxes and discounting on the standardized measure of discounted future net cash flows (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Future net cash flows before income taxes	$ 0	$ 5,479
Future income taxes	0	(1,918)
Future net cash flows	0	3,561
Discount at 10% per annum	0	(1,301)
Standardized measure of discounted future net cash flows	$ 0	$ 2,260	$ 3,527