Oil and Gas Producing Activities (Unaudited) - 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 gross revenues	$ 0	$ 17,938
Future costs:
Lease operating expenses	0	(12,421)
Development costs	0	(38)
Future net cash flows before income taxes	0	5,479
Discount at 10% per annum	0	(2,002)
Discounted future net cash flows before income taxes	0	3,477
Future income taxes, net of discount at 10% per annum	0	(1,217)
Standardized measure of discounted future net cash flows	$ 0	$ 2,260	$ 3,527