Changes in Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Oct. 01, 2017	Sep. 30, 2018	Oct. 01, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss), tax	$ (35)	$ (105)	$ (59)	$ (288)
Foreign currency translation adjustments, tax	0	0	0	0	$ 0	$ 0
Unrealized gains on marketable securities, tax	(902)	1,702	(902)	1,702	1,815	209
Retirement plans prior service benefit, tax	0	(893)	0	(893)	$ (932)	$ (778)
Reclassification of unrealized gains on equity securities, net of tax	17	67	6	173
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
Unrealized Gains (Losses) on Marketable Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, tax			(806)	1,666
Amounts reclassified from accumulated other comprehensive income (loss), tax	(17)	(67)	(6)	(173)
Other comprehensive income (loss), tax			(812)	1,493
Reclassification of income tax effects from the Tax Reform Act, net of tax			(691)
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	$ (18)	$ (38)	(53)	(115)
Other comprehensive income (loss), tax			(53)	$ (115)
Reclassification of income tax effects from the Tax Reform Act, net of tax			(78)
ASU 2016-01 | Foreign Currency Translation Reclassification Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification of unrealized gains on equity securities, net of tax			0
ASU 2016-01 | Unrealized Gains (Losses) on Marketable Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification of unrealized gains on equity securities, net of tax			(902)
ASU 2016-01 | Amortization of Prior Service Credit
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification of unrealized gains on equity securities, net of tax			$ 0