Fair Value of Financial Instruments - Summary of Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financial assets carried at fair value:
Total financial assets	$ 290,940	$ 300,441
Level 1 [Member]
Financial assets carried at fair value:
Total financial assets	12,870	1,724
Level 2 [Member]
Financial assets carried at fair value:
Total financial assets	278,070	298,717
Government money market funds [Member]
Financial assets carried at fair value:
Cash equivalents, fair value disclosure	12,870	1,724
Government money market funds [Member] \| Level 1 [Member]
Financial assets carried at fair value:
Cash equivalents, fair value disclosure	12,870	1,724
Certificate of deposit [Member]
Financial assets carried at fair value:
Short-term investments, available for sale	251
Certificate of deposit [Member] \| Level 2 [Member]
Financial assets carried at fair value:
Short-term investments, available for sale	251
Corporate debt securities, available-for-sale [Member]
Financial assets carried at fair value:
Cash equivalents, fair value disclosure	4,557	21,976
Short-term investments, available for sale	273,262	273,262
Corporate debt securities, available-for-sale [Member] \| Level 2 [Member]
Financial assets carried at fair value:
Cash equivalents, fair value disclosure	4,557	21,976
Short-term investments, available for sale	$ 273,262	273,262
Commercial paper, available-for-sale [Member]
Financial assets carried at fair value:
Short-term investments, available for sale		3,479
Commercial paper, available-for-sale [Member] \| Level 2 [Member]
Financial assets carried at fair value:
Short-term investments, available for sale		$ 3,479

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-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
-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
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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