Summary of Held-to-Maturity Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	$ 27,449	$ 27,051
Unrealized Losses	(115)	(109)
Estimated Fair Value	27,334	26,942
Commercial Paper \| Cash and Cash Equivalents
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	8,810	12,097
Unrealized Losses	(19)	(21)
Estimated Fair Value	8,791	12,076
Commercial Paper \| Short Term Investments
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	16,892	13,334
Unrealized Losses	(96)	(88)
Estimated Fair Value	16,796	13,246
Corporate Bonds \| Cash and Cash Equivalents
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	449	870
Estimated Fair Value	449	870
Corporate Bonds \| Short Term Investments
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	1,298	750
Estimated Fair Value	$ 1,298	$ 750

Amount of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Subparagraph (a)
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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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-Name Accounting Standards Codification
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