Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 103,586	$ 50,868
Gross Unrealized Loss	(326)	(97)
Fair Value	103,260	50,771
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,591	3,758
Fair Value	6,591	3,758
United States government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,944	11,252
Gross Unrealized Loss	(41)	(24)
Fair Value	20,903	11,228
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	76,051	35,858
Gross Unrealized Loss	(285)	(73)
Fair Value	$ 75,766	$ 35,785

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 2
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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