Unaudited: Quarterly Results of Operations, Common Stock Price and Dividends (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) shareholder $ / shares	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) shareholder $ / shares	Sep. 30, 2016 USD ($) $ / shares	Jun. 30, 2016 USD ($) $ / shares	Mar. 31, 2016 USD ($) $ / shares	Dec. 31, 2017 USD ($) shareholder $ / shares	Dec. 31, 2016 USD ($) shareholder $ / shares	Dec. 31, 2015 USD ($) $ / shares
Net sales | $	$ 96,650	$ 100,008	$ 104,176	$ 101,606	$ 93,098	$ 96,273	$ 103,820	$ 100,570	$ 402,440	$ 393,761	$ 377,698
Gross margin | $	39,003	37,039	41,054	38,650	33,522	38,647	39,396	39,011	155,746	150,576	135,776
Net earnings | $	$ 7,233	$ 7,975	$ 10,614	$ 8,749	$ 6,113	$ 8,792	$ 9,400	$ 7,990	$ 34,571	$ 32,295	$ 25,938
Earnings per share:
Basic (in dollars per share)	$ 0.25	$ 0.28	$ 0.37	$ 0.30	$ 0.21	$ 0.30	$ 0.33	$ 0.28
Diluted (in dollars per share)	0.25	0.27	0.36	0.30	0.21	0.30	0.32	0.28
Dividends declared (in dollars per share)	0.13	0.13	0.115	0.115	0.115	0.115	0.1	0.100	$ 0.49	$ 0.43	$ 0.39
Stock price (in dollars per share)	$ 47.80	49.00	39.85	36.75	$ 36.95	33.51	36.52	33.26	$ 47.80	$ 36.95
Number of shareholders | shareholder	909				922				909	922
High
Earnings per share:
Stock price (in dollars per share)	$ 52.10	49.45	41.58	39.85	$ 39.15	37.80	39.36	34.74	$ 52.10	$ 39.15
Low
Earnings per share:
Stock price (in dollars per share)	$ 42.00	$ 39.10	$ 35.15	$ 34.40	$ 29.30	$ 31.90	$ 31.72	$ 26.40	$ 42.00	$ 29.30