Marketable Securities and Cash Equivalents - Summary of Marketable Securities and Cash Equivalents (Detail) $ in Thousands	Sep. 30, 2018 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 74,064
Unrealized Losses	(5)
Fair Value	74,059
Cash Equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	39,007
Fair Value	39,007
Cash Equivalents [Member] \| Money Market Funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	37,708
Fair Value	37,708
Cash Equivalents [Member] \| Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,299
Fair Value	1,299
Marketable Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	35,057
Unrealized Losses	(5)
Fair Value	35,052
Marketable Securities [Member] \| Corporate Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	3,987
Unrealized Losses	(1)
Fair Value	3,986
Marketable Securities [Member] \| Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	19,151
Fair Value	19,151
Marketable Securities [Member] \| Government Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	11,919
Unrealized Losses	(4)
Fair Value	$ 11,915

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
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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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
-SubTopic 10
-Section 45
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