Note 3 - Revenue From Contracts With Customers - Cumulative Effect of Adjustments for New Accounting Pronouncements on Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017
Assets
Inventories	$ 16,052	$ 12,828	$ 13,892
Contract assets	1,575	1,783	2,299
Total current assets	34,597	39,696	41,276
Segment assets	78,957	86,329	87,909	$ 79,577
Liabilities
Current contract liabilities	3,657	4,661	8,829
Refund liabilities	2,518	1,565	695
Total current liabilities	18,447	15,616	18,914
Deferred tax liabilities	1,922	2,241	2,414
Total liabilities	29,242	26,952	30,423
Stockholders' Equity
Accumulated deficit	(116,229)	(106,668)	(108,559)
Total stockholders' equity	49,715	59,377	57,486
Total liabilities and stockholders' equity	$ 78,957	$ 86,329	87,909
Accounting Standards Update 2014-09 [Member]
Assets
Inventories			(1,064)
Contract assets			(516)
Total current assets			(1,580)
Segment assets			(1,580)
Liabilities
Current contract liabilities			(4,168)
Refund liabilities			870
Total current liabilities			(3,298)
Deferred tax liabilities			(173)
Total liabilities			(3,471)
Stockholders' Equity
Accumulated deficit			1,891
Total stockholders' equity			1,891
Total liabilities and stockholders' equity			$ (1,580)