Note 7 - Net Income Per Limited Partner Unit - Earning Per Unit (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net income	$ 6,959	$ 2,408	$ 14,072	$ 8,635
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)
Common units (in shares)	40,811	40,380	40,735	40,331
Restricted and phantom units (in shares)	1,130	1,090	1,004	1,019
Total units (in shares)	41,941	41,470	41,739	41,350
Basic and diluted net income (loss) per common unit (in dollars per share)	$ 0.01	$ (0.09)	$ (0.12)	$ (0.25)