Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Quarterly Financial Information Disclosure [Line Items]
Sales	$ 243,260,000	$ 240,455,000	$ 213,276,000	$ 215,389,000	$ 205,522,000	$ 216,766,000	$ 211,489,000	$ 210,971,000	$ 912,380,000	$ 844,748,000	$ 879,577,000
Gross profit	60,911,000	64,939,000	54,679,000	54,164,000	47,933,000	54,711,000	52,178,000	50,277,000	234,693,000	205,099,000	202,448,000
Net income	$ 3,240,000	$ 16,592,000	$ 12,316,000	$ 12,167,000	$ 7,588,000	$ 13,236,000	$ 10,562,000	$ 8,659,000	$ 44,315,000	$ 40,045,000	$ 43,209,000
Per common share - diluted
Diluted (in dollars per share)	$ 0.27	$ 1.42	$ 1.05	$ 1.05	$ 0.65	$ 1.14	$ 0.92	$ 0.75	$ 3.79	$ 3.46	$ 3.76
Average shares
Diluted (in shares)	11,727	11,708	11,671	11,620	11,608	11,595	11,550	11,507	11,682	11,565	11,482
Dividends per share (in dollars per share)	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.09	$ 0.09	$ 0.09	$ 0.09
Market price of common stock
Estimated foreign earnings and profits tax expense	$ 10,100,000								$ 13,104,000
High
Market price of common stock
Share Price (in dollars per share)	$ 119.5	107.69	92.67	79.50	78.91	68.04	66.01	61.82	$ 119.5	$ 78.91
Low
Market price of common stock
Share Price (in dollars per share)	$ 104.87	$ 86.8	$ 71.72	$ 71.20	$ 59.55	$ 61.49	$ 52.82	$ 48.26	$ 104.87	$ 59.55