Supplemental Information On Oil And Natural Gas Producing Activities (Change Standardized Measure) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Supplemental Information On Oil And Natural Gas Producing Activities
Beginning of the period	$ 49,636	$ 67,852
Sales and transfers of oil and natural gas, net of production costs	(14,758)	(8,700)
Net changes in prices and production costs related to future production	15,036	(7,868)
Changes in development costs	3,854	5,040
Changes in extensions and discoveries	160
Revisions of previous quantity estimates	9,137	(17,924)
Purchases and sales of reserves in place	(11,952)	9,134
Accretion discount	4,964	6,175
Change in production rates, timing, and other	620	(4,073)
Standardized measure of discounted future net cash flows related to proved oil and natural gas reserves	$ 56,697	$ 49,636