CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CONSOLIDATED STATEMENTS OF INCOME [Abstract]
Net sales	$ 1,184,739	$ 1,147,431	$ 983,157
Cost of sales	941,610	882,162	764,314
Gross profit	243,129	265,269	218,843
Selling, general and administrative expenses	160,775	153,145	145,180
Research and development expenses	26,817	24,969	23,676
Income from operations	55,537	87,155	49,987
Other income:
Interest expense	840	1,395	1,611
Interest income	1,302	806	542
Other income	1,218	529	3,055
Income before income taxes	57,217	87,095	51,973
Income taxes	19,627	32,107	20,007
Net income	37,590	54,988	31,966
Net loss attributable to non-controlling interest	(205)	(171)	(831)
Net income attributable to controlling interest	$ 37,795	$ 55,159	$ 32,797
Net income attributable to controlling interest:
Basic (in dollars per share)	$ 1.64	$ 2.40	$ 1.43
Diluted (in dollars per share)	$ 1.63	$ 2.38	$ 1.42
Weighted average number of common shares outstanding:
Basic (in shares)	23,025	22,992	22,934
Diluted (in shares)	23,184	23,142	23,120