Marketable Securities - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2017	Sep. 29, 2018
Schedule Of Available For Sale Securities [Line Items]
Other-than-temporary impairment, net of cost	$ 424,000
Other Nonoperating Income (Expense)
Schedule Of Available For Sale Securities [Line Items]
Net unrealized pre-tax gain in market value		$ 232,000
Net unrealized pre-tax (loss) in market value		$ (201,000)

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

No definition available.

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=109979856&loc=SL6284422-111562

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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