Unaudited: Quarterly Results of Operations, Common Stock Price and Dividends (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($) Stockholder $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) Stockholder $ / shares	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2018 USD ($) Stockholder $ / shares	Dec. 31, 2017 USD ($) Stockholder $ / shares	Dec. 31, 2016 USD ($) $ / shares
Net sales | $	$ 104,413	$ 110,630	$ 113,648	$ 105,041	$ 96,650	$ 100,008	$ 104,176	$ 101,606	$ 433,732	$ 402,440	$ 393,761
Gross margin | $	40,151	43,946	41,504	36,748	39,003	37,039	41,054	38,650	162,349	155,746	150,576
Net earnings | $	$ 11,239	$ 2,851	$ 6,154	$ 7,546	$ 7,233	$ 7,975	$ 10,614	$ 8,749	$ 27,790	$ 34,571	$ 32,295
Earnings per share:
Basic	$ 0.39	$ 0.10	$ 0.21	$ 0.26	$ 0.25	$ 0.28	$ 0.37	$ 0.30	$ 0.96	$ 1.20	$ 1.12
Diluted	0.39	0.10	0.21	0.26	0.25	0.27	0.36	0.30	0.95	1.19	1.11
Dividends declared	0.15	0.15	0.13	0.13	0.130	0.130	0.115	0.115	0.56	0.49	$ 0.43
Stock price	$ 49.21	52.95	44.70	47.15	$ 47.80	49.00	39.85	36.75	$ 49.21	$ 47.80
Number of shareholders | Stockholder	906				909				906	909
High
Earnings per share:
Stock price	$ 57.12	56.40	47.25	51.05	$ 52.10	49.45	41.58	39.85	$ 57.12	$ 52.10
Low
Earnings per share:
Stock price	$ 46.70	$ 50.75	$ 41.00	$ 45.45	$ 42.00	$ 39.10	$ 35.15	$ 34.40	$ 46.70	$ 42.00