Supplementary Financial Information for Oil and Gas Producing Activities (Unaudited) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) MMcf MMBbls	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) MMcf MMBbls	Dec. 31, 2016 USD ($) MMcf MMBbls	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($) MMcf MMBbls	Dec. 31, 2017 USD ($) a $ / MMcf $ / MBbls MMcf MMBbls	Dec. 31, 2016 USD ($) $ / MMcf $ / MBbls MMcf MMBbls	Dec. 31, 2015 USD ($) $ / MMcf $ / MBbls MMcf MMBbls	Dec. 31, 2017 USD ($) MMcf MMBbls	Dec. 31, 2016 USD ($) MMcf MMBbls	Dec. 31, 2015 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												$ 26,786	$ 17,908	$ 25,916
Future estimated production and development costs									$ (11,170)	$ (8,402)	$ (12,019)
Future estimated net revenues									15,616	9,506	13,897
Future estimated income tax expense												(2,592)	(2,458)	(3,642)
Future estimated net cash flows												13,024	7,048	10,255
10% annual discount for estimated timing of cash flows												(6,448)	(2,851)	(4,969)
Standardized measure of discounted future estimated net cash flows	$ 6,576			$ 4,197	$ 4,197			$ 5,286	4,197	5,286	13,288	6,576	4,197	5,286
Changes in standardized measure of discounted future net cash flows
Standardized measure - beginning of year				4,197				5,286	4,197	5,286	13,288
Net change in sales prices and production costs									1,146	(1,403)	(12,535)
Sales of natural gas produced, net of production costs									(1,372)	(928)	(1,188)
Net change due to changes in quantity estimates									1,594	(1,732)	(2,625)
Previously estimated development costs incurred											14
Net change in income taxes									(155)	414	3,147
Accretion of discount									456	571	1,570
Timing of production of reserves and other									710	1,989	3,615
Standardized measure - end of year	6,576				4,197				6,576	4,197	5,286
Results of Operations
Revenues	$ 537	$ 506	$ 553	$ 636	$ 602	$ 554	$ 504	$ 432	2,232	2,092	2,447
Production and operating costs									858	1,164	1,259
Depreciation and depletion									646	868	874
Results of operations before income taxes									728	60	314
Income tax (benefit) expense									145	(93)	(19)
Results of operations (excluding corporate overhead and interest costs)									583	153	333
Costs Incurred
Development costs incurred									$ 2	$ 15	$ 15
Capitalized Costs
Natural gas properties - proved												18,414	18,412	18,398
Less: accumulated depreciation and depletion												11,546	10,900	10,030
Net capitalized costs for natural gas properties												$ 6,868	$ 7,512	$ 8,368
Natural Gas (BCF)
Reserve Quantities
Price (in dollars per Mcf or Bbl) | $ / MMcf									2.81	2.65	2.80
Changes in proved reserves
Proved reserves - beginning of year | MMcf				3,900				5,300	3,900	5,300	6,700
Revisions of previous estimates | MMcf									1,300	(900)	(900)
Production | MMcf									(600)	(500)	(500)
Proved reserves - end of year | MMcf	4,600				3,900				4,600	3,900	5,300
Proved developed reserves - end of year | MMcf												4,600	3,900	5,300
Natural Gas Liquids (MMBLS)
Reserve Quantities
Price (in dollars per Mcf or Bbl) | $ / MBbls									20.23	16.61	14.92
Changes in proved reserves
Proved reserves - beginning of year | MMBbls				0.5				0.7	0.5	0.7	1.0
Revisions of previous estimates | MMBbls									0.3	(0.1)	(0.2)
Production | MMBbls									(0.1)	(0.1)	(0.1)
Proved reserves - end of year | MMBbls	0.7				0.5				0.7	0.5	0.7
Proved developed reserves - end of year | MMBbls												0.7	0.5	0.7