Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue from contracts with customers	$ 74,885	$ 115,902	$ 233,359	$ 208,836
Total revenue	91,756	133,158	279,564	261,311
Operating expense	25,168	27,174	78,326	87,297
General and administrative expense	3,840	4,322	10,495	13,029
Asset impairment expense	83	15	2,316	631
Total costs and expenses	80,754	126,432	255,092	242,303
Gain (loss) on sale of assets	(40)	(63)	1,765	300
Operating income	10,962	6,663	26,237	19,308
Other income (expenses):
Other income			268
Gain on sale of unconsolidated affiliate				2,225
Interest expense	(3,989)	(4,090)	(12,394)	(12,683)
Income before income taxes	6,973	2,573	14,111	8,850
Provision for income taxes	14	165	39	215
Net income	6,959	2,408	14,072	8,635
Allocation of net income for calculation of earnings per unit:
General partner interest in net income	110	39	268	298
Preferred interest in net income	6,278	6,279	18,836	18,836
Net income (loss) available to limited partners	$ 571	$ (3,910)	$ (5,032)	$ (10,499)
Basic and diluted net income (loss) per common unit (in dollars per share)	$ 0.01	$ (0.09)	$ (0.12)	$ (0.25)
Weighted average common units outstanding - basic and diluted (in shares)	40,811	40,380	40,735	40,331
Third Party [Member]
Lease revenue	$ 11,444	$ 11,368	$ 31,026	$ 31,409
Cost of product sales	18,972	50,815	64,069	73,493
Related Party [Member]
Lease revenue	5,427	5,406	15,179	20,584
Cost of product sales	32,691	44,106	99,886	67,853
Service [Member] | Third Party [Member]
Revenue from contracts with customers	15,716	12,743	47,329	44,164
Service [Member] | Related Party [Member]
Revenue from contracts with customers	3,956	5,396	12,257	17,780
Product [Member] | Third Party [Member]
Revenue from contracts with customers	55,213	97,763	173,773	146,892
Product [Member] | Related Party [Member]
Revenue from contracts with customers		$ 482		$ 482