Unaudited: Quarterly Results of Operations, Common Stock Price and Dividends (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014 shareholder	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013 shareholder	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 shareholder	Dec. 31, 2013 shareholder	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012
Net sales	$ 89,339	$ 96,271	$ 95,662	$ 83,496	$ 81,010	$ 92,963	$ 88,341	$ 71,808	$ 364,768	$ 334,122	$ 319,660
Gross margin	30,895	36,465	34,831	28,951	29,003	33,103	29,846	25,037	131,142	116,989	122,246
Net earnings	$ 6,001	$ 10,231	$ 8,814	$ 4,632	$ 6,380	$ 9,049	$ 6,281	$ 2,907	$ 29,678	$ 24,617	$ 28,032
Earnings Per Share, Basic (in dollars per share)	$ 0.42	$ 0.71	$ 0.62	$ 0.33	$ 0.44	$ 0.63	$ 0.44	$ 0.21
Earnings Per Share, Diluted (in dollars per share)	$ 0.42	$ 0.71	$ 0.61	$ 0.32	$ 0.44	$ 0.63	$ 0.44	$ 0.20
Dividends declared (in dollars per share)	$ 0.19	$ 0.19	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.17	$ 0.17
Stock price (in dollars per share)					$ 54.50					$ 54.50		$ 46.50	$ 44.55	$ 53.52
Number of shareholders	948				969				948	969
Common Stock
Stock price (in dollars per share)	$ 59.35	$ 50.45	$ 52.65	$ 55.10					$ 59.35
Common Stock | High
Stock price (in dollars per share)	$ 60.91	$ 54.33	$ 56.96	$ 56.03	$ 56.36				$ 60.91	$ 56.36		$ 49.96	$ 54.28	$ 54.20
Common Stock | Low
Stock price (in dollars per share)	$ 47.46	$ 47.32	$ 46.47	$ 47.90	$ 44.80				$ 47.46	$ 44.80		$ 43.96	$ 41.88	$ 46.82