Reporting Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment operating revenues:
Gathering and transportation lease revenues	$ 14,135		$ 13,148				$ 46,361	$ 38,634
Total revenues	20,916		18,152				60,261	53,727
Segment operating costs:
Lease operating expenses	2,105		1,905				5,885	5,883
Transportation operating expenses	2,752		3,061				8,476	8,979
Production taxes	165		292				489	901
Gain on sale of assets			(238)					(2,626)
Depreciation, depletion and amortization	6,441		6,507				19,044	19,680
Asset impairments		$ 0		$ 0	$ 0	$ 0
Accretion expense	132		123				391	372	$ 497
Total operating expenses	16,183		16,914				48,603	53,322
Segment other income (loss)
Earnings (loss) from equity investments	780		2,313				3,013	9,696
Segment operating income	10,101		8,815				28,989	30,234
Production
Segment operating revenues:
Total revenues	5,061		3,422				8,795	10,162
Segment operating costs:
Lease operating expenses	1,636		1,597				4,643	4,993
Production taxes	165		292				489	901
Gain on sale of assets			(238)					(2,626)
Depreciation, depletion and amortization	1,077		1,202				3,000	3,816
Accretion expense	49		48				149	151
Total operating expenses	2,927		2,901				8,281	7,235
Segment other income (loss)
Segment operating income	2,134		521				514	2,927
Midstream
Segment operating revenues:
Gathering and transportation lease revenues	14,135		13,148				46,361	38,634
Total revenues	15,855		14,730				51,466	43,565
Segment operating costs:
Lease operating expenses	469		308				1,242	890
Transportation operating expenses	2,752		3,061				8,476	8,979
Depreciation, depletion and amortization	5,364		5,305				16,044	15,864
Accretion expense	83		75				242	221
Total operating expenses	8,668		8,749				26,004	25,954
Segment other income (loss)
Earnings (loss) from equity investments	780		2,313				3,013	9,696
Total segment other income (loss)	780		2,313				3,013	9,696
Segment operating income	7,967		8,294				28,475	27,307
Natural gas sales
Segment operating revenues:
Revenues	177		166				543	865
Natural gas sales | Production
Segment operating revenues:
Revenues	177		166				543	865
Oil sales
Segment operating revenues:
Revenues	4,769		2,848				7,841	7,894
Oil sales | Production
Segment operating revenues:
Revenues	4,769		2,848				7,841	7,894
Natural gas liquid product
Segment operating revenues:
Revenues	115		408				411	1,403
Natural gas liquid product | Production
Segment operating revenues:
Revenues	115		408				411	1,403
Gathering and transportation
Segment operating revenues:
Revenues	1,720		1,582				5,105	4,931
Gathering and transportation | Midstream
Segment operating revenues:
Revenues	$ 1,720		$ 1,582				$ 5,105	$ 4,931