CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Net sales	[1]	$ 656,314,000	$ 641,443,000	$ 610,911,000
Cost of sales		406,829,000	406,081,000	376,158,000
Gross margin		249,485,000	235,362,000	234,753,000
Selling, general and administrative expenses		136,317,000	131,463,000	125,244,000
Research and development expenses		28,582,000	27,644,000	22,878,000
Restructuring and impairment charges		734,000	0	5,390,000
Operating income		83,852,000	76,255,000	81,241,000
Equity income in unconsolidated joint ventures		4,146,000	2,890,000	4,123,000
Interest income (expense), net		(3,930,000)	(4,480,000)	(2,946,000)
Other income (expense), net		(1,788,000)	(8,492,000)	(1,194,000)
Income before income taxes		82,280,000	66,173,000	81,224,000
Income tax expense		33,997,000	19,853,000	27,812,000
Net income		$ 48,283,000	$ 46,320,000	$ 53,412,000
Earnings per share:
Basic earnings per share (in dollars per share)		$ 2.68	$ 2.52	$ 2.94
Diluted earnings per share (in dollars per share)		$ 2.65	$ 2.48	$ 2.86
Weighted average shares used in computing earnings per share:
Basic earnings per share (in shares)		17,991	18,371	18,177
Diluted earnings per share (in shares)		18,223	18,680	18,698

[1] Net sales are allocated to countries based on the location of the customer.