Per Share Amounts and Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Apr. 01, 2017
Equity [Abstract]
Net loss	$ (6,627)	$ (3,887)
Weighted-average shares outstanding, basic	25,229,000	24,967,000
Weighted-average shares, partially-paid share subscriptions	189,000	208,000
Basic weighted average shares	25,418,000	25,175,000
Incremental shares from assumed exercise of stock subscription purchase rights	146,000	145,000
Incremental shares from assumed exercise of stock options and awards	912,000	896,000
Diluted weighted-average shares	26,476,000	26,216,000
Net loss per share--basic and diluted	$ (0.26)	$ (0.15)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at January 1, 2018	24,220,859
Shares purchased	(301,064)
Shares sold	217,325
Stock subscription offering -- cash purchases	12,670
Options and awards exercised	12,649
Shares outstanding at March 31, 2018	24,162,439
Employee and director options exercisable	786,815
Partially-paid subscriptions	757,040
Purchase rights outstanding	302,797
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	(261,959)
Shares sold	0
Stock subscription offering -- cash purchases	12,670
Options and awards exercised	12,649
Shares outstanding at March 31, 2018	17,517,192
Redeemable shares [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at January 1, 2018	6,467,027
Shares purchased	(39,105)
Shares sold	217,325
Stock subscription offering -- cash purchases	0
Options and awards exercised	0
Shares outstanding at March 31, 2018	6,645,247