Per Share Amounts and Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 04, 2015	Jun. 28, 2014	Jul. 04, 2015	Jun. 28, 2014	May. 19, 2015	Dec. 31, 2014
Equity [Abstract]
Net income	$ 15,932	$ 14,046	$ 12,404	$ 10,035
Weighted-average shares outstanding, basic	13,099,000	13,360,000	13,146,000	13,419,000
Weighted-average shares, partially-paid share subscriptions	115,000	126,000	229,000	252,000
Basic weighted average shares	13,214,000	13,486,000	13,375,000	13,671,000
Incremental shares from assumed exercise of stock subscription purchase rights	61,000	44,000	62,000	44,000
Incremental shares from assumed exercise of stock options and awards	390,000	350,000	398,000	355,000
Diluted weighted-average shares	13,665,000	13,880,000	13,835,000	14,070,000
Basic	$ 1.21	$ 1.04	$ 0.93	$ 0.73
Diluted	$ 1.17	$ 1.01	$ 0.90	$ 0.71
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at January 1, 2015			13,164,933
Shares purchased			(386,138)
Shares sold			197,749
Stock subscription offering -- cash purchases			18,774
Options and awards exercised			41,832
Change in shares outstanding			(127,783)
Shares outstanding at July 4, 2015	13,037,150		13,037,150
Common stock, par value	$ 1.00		$ 1.00
Common stock, shares authorized	48,000,000		48,000,000		48,000,000	24,000,000
Preferred stock, shares authorized	4,000,000		4,000,000
Employee and director options exercisable	406,284		406,284
Partially-paid subscriptions	459,830		459,830
Purchase rights outstanding	184,239		184,239