Note 18 - Quarterly Financial Data (Unaudited) - Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from Contract with Customer, Excluding Assessed Tax, Total	$ 26,956	$ 26,787	$ 30,548	$ 21,264	$ 29,388	$ 27,184	$ 33,462	$ 23,386	$ 105,555	$ 113,420	$ 103,379
Cost of product revenue	7,001	8,282	8,152	7,845	8,716	6,250	6,315	6,083	31,280	27,364	24,027
Gross profit on product revenue	19,949	18,499	22,390	13,413	20,168	20,928	22,025	17,298
Net income	$ 7,717	$ 7,599	$ 10,092	$ (6,686)	$ 8,067	$ 6,887	$ 11,369	$ 5,493	$ 18,722	$ 31,816	$ 32,547
Basic net income per share (in dollars per share)	$ 0.54	$ 0.53	$ 0.69	$ (0.46)	$ 0.55	$ 0.47	$ 0.78	$ 0.38	$ 1.30	$ 2.18	$ 2.22
Basic common shares outstanding (in shares)	14,168	14,237	14,652	14,679	14,596	14,579	14,588	14,576
Diluted net income per share (in dollars per share)	$ 0.54	$ 0.53	$ 0.68	$ (0.46)	$ 0.53	$ 0.46	$ 0.76	$ 0.37	$ 1.27	$ 2.11	$ 2.15
Diluted common shares outstanding (in shares)	14,299	14,377	14,915	14,679	15,141	15,115	15,044	15,043	14,689	15,068	15,116
Product [Member]
Revenue from Contract with Customer, Excluding Assessed Tax, Total	$ 26,950	$ 26,781	$ 30,542	$ 21,258	$ 28,884	$ 27,178	$ 28,340	$ 23,381	$ 105,531	$ 107,783	$ 102,932