Supplementary Financial Information for Oil and Gas Producing Activities (Unaudited) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) a $ / MMcf $ / MBbls MMcf MMBbls	Dec. 31, 2017 USD ($) $ / MMcf $ / MBbls MMcf MMBbls	Dec. 31, 2016 USD ($) $ / MMcf $ / MBbls MMcf MMBbls	Dec. 31, 2018 USD ($) MMcf MMBbls	Dec. 31, 2017 USD ($) MMcf MMBbls	Dec. 31, 2016 USD ($) MMcf MMBbls
Reserve Quantities
Area of land (in acres) | a	3,800
Area of land under a lease (in acres) | a	538
Unaudited standardized measure of discounted future net cash flows
Future estimated gross revenues				$ 25,759	$ 26,786	$ 17,908
Future estimated production and development costs	$ (10,095)	$ (11,170)	$ (8,402)
Future estimated net revenues	15,664	15,616	9,506
Future estimated income tax expense				(2,479)	(2,592)	(2,458)
Future estimated net cash flows				13,185	13,024	7,048
10% annual discount for estimated timing of cash flows				(6,695)	(6,448)	(2,851)
Standardized measure of discounted future estimated net cash flows	6,576	4,197	5,286	6,490	6,576	4,197
Changes in standardized measure of discounted future net cash flows
Standardized measure - beginning of year	6,576	4,197	5,286
Net change in sales prices and production costs	1,675	1,146	(1,403)
Sales of natural gas produced, net of production costs	(1,654)	(1,372)	(928)
Net change due to changes in quantity estimates	(590)	1,594	(1,732)
Net change in income taxes	414	(155)	414
Accretion of discount	555	456	571
Timing of production of reserves and other	(486)	710	1,989
Standardized measure - end of year	6,490	6,576	4,197
Results of Operations
Revenues	2,513	2,232	2,092
Production and operating costs	859	858	1,164
Depreciation and depletion	650	646	868
Results of operations before income taxes	1,004	728	60
Income tax (benefit) expense	155	145	(93)
Results of operations (excluding corporate overhead and interest costs)	$ 849	583	153
Costs Incurred
Development costs incurred		$ 2	$ 15
Capitalized Costs
Natural gas properties - proved				18,414	18,414	18,412
Less: accumulated depreciation and depletion				12,198	11,546	10,900
Net capitalized costs for natural gas properties				$ 6,216	$ 6,868	$ 7,512
Natural Gas (BCF)
Reserve Quantities
Price (in dollars per Mcf or Bbl) | $ / MMcf	2.82	2.81	2.65
Changes in proved reserves
Proved reserves - beginning of year | MMcf	4,600	3,900	5,300
Revisions of previous estimates | MMcf	(300)	1,300	(900)
Production | MMcf	(400)	(600)	(500)
Proved reserves - end of year | MMcf	3,900	4,600	3,900
Proved developed reserves - end of year | MMcf				3,900	4,600	3,900
Natural Gas Liquids (MMBLS)
Reserve Quantities
Price (in dollars per Mcf or Bbl) | $ / MBbls	25.57	20.23	16.61
Changes in proved reserves
Proved reserves - beginning of year | MMBbls	0.7	0.5	0.7
Revisions of previous estimates | MMBbls	0.0	0.3	(0.1)
Production | MMBbls	(0.1)	(0.1)	(0.1)
Proved reserves - end of year | MMBbls	0.6	0.7	0.5
Proved developed reserves - end of year | MMBbls				0.6	0.7	0.5