Note 2 - Revenue Recognition - Adoption of ASC 606 (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Property, plant and equipment	$ 111,289		$ 111,289		$ 107,743	$ 106,716
Less accumulated depreciation and amortization	(52,600)		(52,600)		(53,612)	(53,064)
Net property, plant and equipment	58,689		58,689		54,131	53,652
Deferred revenue	3,331		3,331		871	297
Deferred income taxes	2,937		2,937		2,415	2,440
Retained earnings	103,259		103,259		97,492	$ 97,562
Net sales	49,019	$ 37,886	91,949	$ 74,939
Cost of sales	36,033	27,945	68,779	55,482
Gross profit	12,986	$ 9,941	23,170	$ 19,457
Accounting Standards Update 2014-09 [Member]
Property, plant and equipment					1,027
Less accumulated depreciation and amortization					(548)
Net property, plant and equipment					479
Deferred revenue					574
Deferred income taxes					(25)
Retained earnings					$ (70)
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Property, plant and equipment	110,150		110,150
Less accumulated depreciation and amortization	(51,808)		(51,808)
Net property, plant and equipment	58,342		58,342
Deferred revenue	2,909		2,909
Deferred income taxes	2,962		2,962
Retained earnings	103,309		103,309
Net sales			91,797
Cost of sales			68,647
Gross profit			23,150
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Property, plant and equipment	1,139		1,139
Less accumulated depreciation and amortization	(792)		(792)
Net property, plant and equipment	347		347
Deferred revenue	422		422
Deferred income taxes	(25)		(25)
Retained earnings	$ (50)		(50)
Net sales			152
Cost of sales			132
Gross profit			$ 20