Investments - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2018	Dec. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Investments maximum maturity period	5 years
Unrealized losses, net of unrealized gains, on the investments in the bond portfolio	$ 1,308	$ 223

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