Reporting Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment operating revenues:
Gathering and transportation lease revenues	$ 13,168		$ 25,486
Total revenues	17,039	$ 25,029	35,575	$ 50,836
Segment operating costs:
Lease operating expenses	2,007	3,881	3,978	8,864
Transportation operating expenses	3,071	3,032	5,918	6,328
Cost of sales		40		77
Production taxes	287	353	609	826
Gain on sale of assets	(2,388)		(2,388)
Depreciation, depletion and amortization	6,545	8,937	13,173	21,118
Asset impairments				4,688
Accretion expense	123	240	249	498	$ 773
Total operating expenses	17,911	23,616	36,408	55,681
Segment other income (loss)
Earnings (loss) from equity investments	3,111	1,042	7,383	1,524
Segment operating income	10,505	9,588	21,419	9,961
Production
Segment operating revenues:
Total revenues	2,210	10,853	6,740	25,449
Segment operating costs:
Lease operating expenses	1,644	3,648	3,396	8,372
Cost of sales		40		77
Production taxes	287	353	609	826
Gain on sale of assets	(2,388)		(2,388)
Depreciation, depletion and amortization	1,251	2,924	2,614	6,205
Asset impairments				4,688
Accretion expense	49	172	103	364
Total operating expenses	843	7,137	4,334	20,532
Segment other income (loss)
Earnings (loss) from equity investments		27		(109)
Total segment other income		27		(109)
Segment operating income	1,367	3,743	2,406	4,808
Midstream
Segment operating revenues:
Gathering and transportation lease revenues	13,168		25,486
Total revenues	14,829	14,176	28,835	25,387
Segment operating costs:
Lease operating expenses	363	233	582	492
Transportation operating expenses	3,071	3,032	5,918	6,328
Depreciation, depletion and amortization	5,294	6,013	10,559	14,913
Accretion expense	74	68	146	134
Total operating expenses	8,802	9,346	17,205	21,867
Segment other income (loss)
Earnings (loss) from equity investments	3,111	1,015	7,383	1,633
Total segment other income	3,111	1,015	7,383	1,633
Segment operating income	9,138	5,845	19,013	5,153
Natural gas
Segment operating revenues:
Revenue from contracts with customers	226	2,252	699	5,031
Natural gas | Production
Segment operating revenues:
Revenue from contracts with customers	226	2,252	699	5,031
Oil
Segment operating revenues:
Revenue from contracts with customers	1,584	8,109	5,046	19,459
Oil | Production
Segment operating revenues:
Revenue from contracts with customers	1,584	8,109	5,046	19,459
Natural gas liquid
Segment operating revenues:
Revenue from contracts with customers	400	492	995	959
Natural gas liquid | Production
Segment operating revenues:
Revenue from contracts with customers	400	492	995	959
Gathering and transportation
Segment operating revenues:
Revenue from contracts with customers	1,661	14,176	3,349	25,387
Gathering and transportation | Midstream
Segment operating revenues:
Revenue from contracts with customers	$ 1,661	$ 14,176	$ 3,349	$ 25,387