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	Apr. 30, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Future net cash flows before income taxes	$ 0	$ 5,479	$ 8,413
Future income taxes	0	(1,918)	(2,945)
Future net cash flows	0	3,561	5,468
Discount at 10% per annum	0	(1,301)	(1,941)
Standardized measure of discounted future net cash flows	$ 0	$ 2,260	$ 3,527	$ 15,744