Note 14 - Selected Interim Financial Data (Unaudited) - Summary of Quarterly Financial Information 1 (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues	$ 30,627	$ 29,440	$ 28,446	$ 27,652	$ 27,881	$ 27,948	$ 27,300	$ 26,868
Income from operations	8,022	8,510	8,823	8,141	8,503	9,109	8,316	6,530
Net loss	(2,650)	(182)	(1,158)	(133)	(1,566)	1,251	308	(8,994)	$ (4,123)	$ (9,001)	$ (6,001)
Net loss attributable to common stockholders	$ (1,071)	$ (73)	$ (467)	$ (54)	$ (632)	$ 505	$ 124	$ (3,630)	$ (1,665)	$ (3,633)	$ (2,365)
Basic and diluted net loss per share (in dollars per share)	$ (0.06)	$ (0.01)	$ (0.03)	$ (0.01)	$ (0.04)	$ 0.02	$ 0	$ (0.21)	$ (0.11)	$ (0.22)	$ (0.15)