Per Share Amounts and Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Equity [Abstract]
Net income	$ 21,714	$ 14,223	$ 15,087	$ 10,336
Weighted-average shares outstanding, basic	22,926,000	24,797,000	24,077,000	24,582,000
Weighted-average shares, partially-paid share subscriptions	195,000	201,000	362,000	402,000
Basic weighted average shares	23,121,000	24,998,000	24,439,000	24,984,000
Incremental shares from assumed exercise of stock subscription purchase rights	140,000	141,000	143,000	143,000
Incremental shares from assumed exercise of stock options and awards	1,046,000	846,000	979,000	871,000
Diluted weighted-average shares	24,307,000	25,985,000	25,561,000	25,998,000
Net income per share basic	$ 0.94	$ 0.57	$ 0.62	$ 0.41
Net income per share diluted	$ 0.89	$ 0.55	$ 0.59	$ 0.40
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at January 1, 2018			24,220,859
Shares purchased			(1,361,942)
Shares sold			604,366
Stock subscription offering -- cash purchases			51,296
Options and awards exercised			108,850
Shares outstanding at June 30, 2018	23,623,429		23,623,429
Common stock, shares subscribed but unissued	723,914		723,914
Purchase rights, outstanding	289,391		289,391
Common shares, net of treasury shares [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at January 1, 2018			17,753,832
Shares purchased			(828,495)
Shares sold			369,171
Stock subscription offering -- cash purchases			51,296
Options and awards exercised			108,850
Shares outstanding at June 30, 2018	17,454,654		17,454,654
Redeemable shares [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at January 1, 2018			6,467,027
Shares purchased			(533,447)
Shares sold			235,195
Stock subscription offering -- cash purchases			0
Options and awards exercised			0
Shares outstanding at June 30, 2018	6,168,775		6,168,775