Changes in Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2019	Sep. 30, 2018	Sep. 29, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss), tax	$ (110)	$ (35)	$ (157)	$ (59)
Reclassification of unrealized gains on equity securities, net of tax	(99)	(17)	(125)	(6)
Foreign currency translation adjustments, tax	0	0	0	0	$ 0	$ 0
Unrealized (losses) gains on marketable securities, tax	1,116	(590)	1,116	(590)	(521)	1,815
Retirement plans prior service benefit, tax	(1,113)	(1,063)	(1,113)	(1,063)	$ (1,081)	$ (932)
Foreign Currency Translation Reclassification Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, tax			0	0
Amounts reclassified from accumulated other comprehensive income (loss), tax			0	0
Other comprehensive income (loss), tax			0	0
Reclassification of income tax effects from the Tax Reform Act, net of tax				0
Reclassification of unrealized gains on equity securities, net of tax				0
Unrealized (Losses) Gains on Marketable Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, tax			1,762	(806)
Amounts reclassified from accumulated other comprehensive income (loss), tax	(99)	(17)	(125)	(6)
Other comprehensive income (loss), tax			1,637	(812)
Reclassification of income tax effects from the Tax Reform Act, net of tax				(691)
Reclassification of unrealized gains on equity securities, net of tax				(902)
Amortization of Prior Service Credit
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, tax			0	0
Amounts reclassified from accumulated other comprehensive income (loss), tax	$ (11)	$ (18)	(32)	(53)
Other comprehensive income (loss), tax			$ (32)	(53)
Reclassification of income tax effects from the Tax Reform Act, net of tax				(78)
Reclassification of unrealized gains on equity securities, net of tax				$ 0