Reclassifications Out of Accumulated Other Comprehensive Income (Loss) to Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Oct. 01, 2017	Sep. 30, 2018	Oct. 01, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income and interest expense	$ (6,213)	$ (4,517)	$ (17,620)	$ (11,329)
Reclassifications, net of tax			(1,227)	468
Unrealized Gains (Losses) on Marketable Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax			(1,411)	264
Amortization of Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax	61	68	184	204
Accumulated Other Comprehensive Income (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax	118	155	(1,227)	468
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains (Losses) on Marketable Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income and interest expense	$ 57	$ 87	$ (1,411)	$ 264