Supplementary Balance Sheet Information - Aggregate Fair Value and Unrealized Holding Gains and Losses by Major Security Type (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Long-Term Marketable Securities [Member] \| Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Maximum maturity period of securities	5 years	5 years

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.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration