Unaudited: Quarterly Results of Operations, Common Stock Price and Dividends (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($) Stockholder $ / shares	Sep. 30, 2020 USD ($) $ / shares	Jun. 30, 2020 USD ($) $ / shares	Mar. 31, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($) Stockholder $ / shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2020 USD ($) Stockholder $ / shares	Dec. 31, 2019 USD ($) Stockholder $ / shares	Dec. 31, 2018 USD ($) $ / shares
Net sales | $	$ 112,329	$ 113,587	$ 91,119	$ 108,508	$ 107,556	$ 108,646	$ 103,542	$ 104,881	$ 425,544	$ 424,625	$ 433,732
Gross margin | $	44,055	45,023	35,850	43,322	41,125	41,670	40,276	40,457	168,249	163,528	162,349
Net earnings | $	$ 13,094	$ 14,861	$ 9,534	$ 11,854	$ 12,274	$ 12,721	$ 11,358	$ 10,824	$ 49,343	$ 47,177	$ 27,790
Earnings per share:
Basic	$ 0.45	$ 0.51	$ 0.33	$ 0.41	$ 0.42	$ 0.44	$ 0.39	$ 0.37	$ 1.70	$ 1.63	$ 0.96
Diluted	0.45	0.51	0.33	0.41	0.42	0.44	0.39	0.37	1.69	1.61	0.95
Dividends declared	0.18	0.18	0.17	0.17	0.17	0.17	0.15	0.15	0.70	0.64	$ 0.56
Stock price	$ 94.06	65.37	62.92	53.60	$ 64.93	53.70	59.69	55.64	$ 94.06	$ 64.93
Number of shareholders | Stockholder	656				790				656	790
High
Earnings per share:
Stock price	$ 96.00	68.25	68.01	70.83	$ 66.64	60.52	60.28	61.57	$ 96.00	$ 66.64
Low
Earnings per share:
Stock price	$ 64.96	$ 59.53	$ 47.00	$ 41.50	$ 50.67	$ 49.66	$ 51.56	$ 47.59	$ 64.96	$ 50.67