Investments - Additional Information (Detail) - USD ($)	9 Months Ended
Sep. 26, 2015	Dec. 27, 2014
Schedule of Available-for-sale Securities [Line Items]
Investments maximum maturity period	5 years
Unrealized gains, net of unrealized losses, on the investments in the bond portfolio	$ 271,000	$ 163,000

Amount before tax of unrealized gain (loss) in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

No definition available.

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant