Per Share Amounts and Common Shares Outstanding (Details) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jul. 01, 2017 USD ($) $ / shares shares	Jul. 02, 2016 USD ($) $ / shares shares	Jul. 01, 2017 USD ($) $ / shares shares	Jul. 02, 2016 USD ($) $ / shares shares	Dec. 31, 2016 $ / shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at January 1, 2017				24,923,140
Shares purchased				(942,035)
Shares sold				449,298
Stock subscription offering -- cash purchases				41,781
Options and awards exercised				105,819
Shares outstanding at July 1, 2017		24,578,003		24,578,003
Stock split, conversion ratio				2
Net income | $		$ 14,223	$ 13,309	$ 10,336	$ 9,121
Weighted-average shares outstanding, basic	[1]	24,797,000	25,528,000	24,582,000	25,534,000
Weighted-average shares, partially-paid share subscriptions	[1]	201,000	212,000	402,000	424,000
Basic weighted average shares	[1]	24,998,000	25,740,000	24,984,000	25,958,000
Incremental shares from assumed exercise of stock subscription purchase rights	[1]	141,000	136,000	143,000	136,000
Incremental shares from assumed exercise of stock options and awards	[1]	846,000	822,000	871,000	874,000
Diluted weighted-average shares	[1]	25,985,000	26,698,000	25,998,000	26,968,000
Net income per share basic | $ / shares	[1]	$ 0.57	$ 0.52	$ 0.41	$ 0.35
Net income per share diluted | $ / shares	[1]	$ 0.55	$ 0.50	$ 0.40	$ 0.34
Employee and director options exercisable	[1]	869,302		869,302
Partially-paid subscriptions	[1]	804,048		804,048
Purchase rights outstanding	[1]	319,956		319,956
Common stock, par value | $ / shares		$ 1.00		$ 1.00		$ 1.00
Permanent equity [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at January 1, 2017				17,866,236
Shares purchased				(351,033)
Shares sold				184,651
Stock subscription offering -- cash purchases				41,781
Options and awards exercised				105,819
Shares outstanding at July 1, 2017		17,847,454		17,847,454
Temporary equity [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at January 1, 2017				7,056,904
Shares purchased				(591,002)
Shares sold				264,647
Stock subscription offering -- cash purchases				0
Options and awards exercised				0
Shares outstanding at July 1, 2017		6,730,549		6,730,549

[1] Adjusted for two-for-one stock split