Oil and Gas Producing Activities (Unaudited) - Principal sources of changes in the standardized measure of discounted future net flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Roll Forward]
Beginning of year	$ 2,260	$ 3,527
Sale of crude oil and natural gas reserves	(2,732)	(350)
Net change in prices and production costs	0	(1,391)
New field discoveries and extensions, net of future production costs	94	275
Sales of crude oil and natural gas produced, net of production costs	(476)	87
Net change due to revisions in quantity estimates	0	181
Accretion of discount	130	194
Production rate changes and other	(493)	(945)
Net change in income taxes	1,217	682
End of year	$ 0	$ 2,260