Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands		Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Noncontrolling Interest [Member]	Total
Balance at Dec. 30, 2017		$ 28,489	$ 127,663	$ 150,726	$ (17,787)		$ 289,091
Net income (loss)				19,770			19,770
Changes in cumulative translation adjustment					(2,960)		(2,960)
Adjustments related to postretirement benefits, net of tax					41		41
Cash dividends				(3,513)			(3,513)
Exercise of stock options		50	464				514
Shares issued under ESPP		41	721				762
Shares issued for restricted stock units vested		468	(468)
Repurchase and retirement of stock		(165)	(3,749)				(3,914)
Share-based compensation expense			3,617				3,617
Cumulative effect of accounting change at Dec. 30, 2017	[1]			1,057			1,057
Changes in unrealized gains and losses on investments, net of tax					(2)		(2)
Balance at Jun. 30, 2018		28,883	128,248	168,040	(20,708)		304,463
Balance at Mar. 31, 2018		28,796	125,448	158,124	(14,016)		298,352
Net income (loss)				11,648			11,648
Changes in cumulative translation adjustment					(6,814)		(6,814)
Adjustments related to postretirement benefits, net of tax					114		114
Cash dividends				(1,732)			(1,732)
Exercise of stock options		31	280				311
Shares issued under ESPP		41	721				762
Shares issued for restricted stock units vested		20	(20)
Repurchase and retirement of stock		(5)	(129)				(134)
Share-based compensation expense			1,948				1,948
Changes in unrealized gains and losses on investments, net of tax					8		8
Balance at Jun. 30, 2018		28,883	128,248	168,040	(20,708)		304,463
Balance at Dec. 29, 2018		40,763	419,690	111,670	(25,880)	(299)	545,944	[2]
Net income (loss)				(42,046)			(42,046)
Changes in cumulative translation adjustment					(1,574)	(4)	(1,578)
Adjustments related to postretirement benefits, net of tax					435		435
Cash dividends				(4,914)			(4,914)
Exercise of stock options		15	107				122
Shares issued under ESPP		64	743				807
Shares issued for restricted stock units vested		394	(394)
Repurchase and retirement of stock		(136)	(1,892)				(2,028)
Share-based compensation expense			7,355				7,355
Cumulative effect of accounting change at Dec. 29, 2018	[1]			10,352			10,352
Noncontrolling interest				53		(53)
Changes in unrealized gains and losses on investments, net of tax
Balance at Jun. 29, 2019		41,100	425,609	75,115	(27,019)	(356)	514,449
Balance at Mar. 30, 2019		41,015	421,381	96,938	(30,664)	(356)	528,314
Net income (loss)				(19,359)			(19,359)
Changes in cumulative translation adjustment					3,684		3,684
Adjustments related to postretirement benefits, net of tax					(39)		(39)
Cash dividends				(2,464)			(2,464)
Exercise of stock options		5	39				44
Shares issued under ESPP		64	743				807
Shares issued for restricted stock units vested		29	(29)
Repurchase and retirement of stock		(13)	(187)				(200)
Share-based compensation expense			3,662				3,662
Changes in unrealized gains and losses on investments, net of tax
Balance at Jun. 29, 2019		$ 41,100	$ 425,609	$ 75,115	$ (27,019)	$ (356)	$ 514,449

[1] Cumulative effect of accounting change relates to our adoption of ASU 2014-09.

[2] Derived from December 29, 2018 audited financial statements