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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Product revenue	$ 28,884	$ 27,178	$ 28,340	$ 23,381	$ 28,296	$ 25,783	$ 26,575	$ 22,278	$ 107,783	$ 102,932	$ 87,696
Total revenue	29,388	27,184	33,462	23,386	28,726	25,789	26,581	22,283	113,420	103,379	92,999
Cost of product revenue	8,716	6,250	6,315	6,083	7,539	4,998	6,065	5,425	27,364	24,027	21,053
Gross profit on product revenue	20,168	20,928	22,025	17,298	20,757	20,785	20,510	16,853
Net income	$ 8,067	$ 6,887	$ 11,369	$ 5,493	$ 8,085	$ 8,952	$ 8,615	$ 6,895	$ 31,816	$ 32,547	$ 30,758
Net income (in dollars per share)	$ 0.55	$ 0.47	$ 0.78	$ 0.38	$ 0.56	$ 0.61	$ 0.59	$ 0.46	$ 2.18	$ 2.22	$ 2.06
Basic common shares outstanding (in shares)	14,596	14,579	14,588	14,576	14,538	14,625	14,679	14,875
Diluted net income per share (in dollars per share)	$ 0.53	$ 0.46	$ 0.76	$ 0.37	$ 0.54	$ 0.59	$ 0.57	$ 0.45	$ 2.11	$ 2.15	$ 2.01
Diluted common shares outstanding (in shares)	15,141	15,115	15,044	15,043	14,979	15,077	15,111	15,307	15,068	15,116	15,321