Note 3 - Revenue From Contracts With Customers - Cumulative Effect of Adjustments for New Accounting Pronouncements on Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017
Assets
Inventories	$ 14,684	$ 12,828	$ 13,892
Contract assets	1,058	1,783	2,299
Total current assets	36,976	39,696	41,276
Segment assets	81,517	86,329	87,909	$ 79,706
Liabilities
Current contract liabilities	4,690	4,661	8,829
Refund liabilities	2,137	1,565	695
Total current liabilities	19,405	15,616	18,914
Deferred tax liabilities	1,941	2,241	2,414
Total liabilities	30,268	26,952	30,423
Stockholders' Equity
Accumulated deficit	(114,695)	(106,668)	(108,559)
Total stockholders' equity	51,249	59,377	57,486
Total liabilities and stockholders' equity	$ 81,517	$ 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)