Unaudited: Quarterly Results of Operations, Common Stock Price and Dividends (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) Stockholder $ / shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) Stockholder $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2019 USD ($) Stockholder $ / shares	Dec. 31, 2018 USD ($) Stockholder $ / shares	Dec. 31, 2017 USD ($) $ / shares
Net sales | $	$ 107,556	$ 108,646	$ 103,542	$ 104,881	$ 104,413	$ 110,630	$ 113,648	$ 105,041	$ 424,625	$ 433,732	$ 402,440
Gross margin | $	41,125	41,670	40,276	40,457	40,151	43,946	41,504	36,748	163,528	162,349	155,746
Net earnings | $	$ 12,274	$ 12,721	$ 11,358	$ 10,824	$ 11,239	$ 2,851	$ 6,154	$ 7,546	$ 47,177	$ 27,790	$ 34,571
Earnings per share:
Basic	$ 0.42	$ 0.44	$ 0.39	$ 0.37	$ 0.39	$ 0.10	$ 0.21	$ 0.26	$ 1.63	$ 0.96	$ 1.20
Diluted	0.42	0.44	0.39	0.37	0.39	0.10	0.21	0.26	1.61	0.95	1.19
Dividends declared	0.17	0.17	0.15	0.15	0.15	0.15	0.13	0.13	0.64	0.56	$ 0.49
Stock price	$ 64.93	53.70	59.69	55.64	$ 49.21	52.95	44.70	47.15	$ 64.93	$ 49.21
Number of shareholders | Stockholder	790				906				790	906
High
Earnings per share:
Stock price	$ 66.64	60.52	60.28	61.57	$ 57.12	56.40	47.25	51.05	$ 66.64	$ 57.12
Low
Earnings per share:
Stock price	$ 50.67	$ 49.66	$ 51.56	$ 47.59	$ 46.70	$ 50.75	$ 41.00	$ 45.45	$ 50.67	$ 46.70