Supplemental Financial Data - Oil and Gas Producing Activities (unaudited) (Details 6) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Principal Sources of Change in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Abstract]
Standardized measure of discounted future net cash flows, beginning of year	$ 57,082	$ 44,711
Sales of oil and gas, net of production costs and taxes	(25,681)	(10,038)
Price revisions	133,789	17,588
Extensions, discoveries and improved recovery, less related costs		298
Changes in estimated future development costs	(32,711)	(324)
Development costs incurred during the period	926	804
Quantity revisions	(23,484)	11,196
Accretion of discount	5,708	4,471
Net changes in future income taxes	(89,117)	(7,425)
Purchases of reserves-in-place	391,877
Sales of reserves-in-place
Changes in production rate timing and other	(26,321)	(4,199)
Standardized measure of discounted future net cash flows, end of year	$ 392,068	$ 57,082