Additional Balance Sheet and Cash Flow Information (Narrative) (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 26, 2017	Mar. 27, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Gross unrealized gains (losses)	$ (16)	$ (71)
Gross unrealized gains (losses), net of taxes	$ (10)	$ (45)

This item represents the pretax change in net unrealized holding gain (loss) on available-for-sale securities that has been included in accumulated other comprehensive income, a separate component of shareholders' equity, during the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563

Name:	us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration