Oil and Gas Producing Activities (Unaudited) - Principal sources of changes in the standardized measure of discounted future net flows (Details) - USD ($) $ in Thousands	4 Months Ended	12 Months Ended
	Apr. 30, 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]
Beginning of year	$ 2,260	$ 3,527	$ 15,744
Sale of crude oil and natural gas reserves	(2,732)	(350)	(54)
Net change in prices and production costs	0	(1,391)	(17,622)
New field discoveries and extensions, net of future production costs	94	275	292
Sales of crude oil and natural gas produced, net of production costs	(476)	87	1,038
Net change due to revisions in quantity estimates	0	181	38
Accretion of discount	130	194	1,116
Production rate changes and other	(493)	(945)	(3,603)
Net change in income taxes	1,217	682	6,578
End of year	$ 0	$ 2,260	$ 3,527