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Significant Accounting Policies - Estimated Fair Value of Assets and Liability (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Asset:
Asset, fair value	$ 420,014	$ 473,882
Money Market Fund
Asset:
Asset, fair value	116,456	88,266
U.S. Treasury Shares
Asset:
Asset, fair value	22,214	48,356
Corporate Bonds
Asset:
Asset, fair value	95,771	179,864
Commercial Paper
Asset:
Asset, fair value	126,832	106,650
Agency Securities
Asset:
Asset, fair value	6,748	20,782
Repo Securities
Asset:
Asset, fair value	9,000	10,000
Asset Backed Securities
Asset:
Asset, fair value	9,895	8,599
Certificates of Deposit
Asset:
Asset, fair value	21,575	6,125
Non-US Government
Asset:
Asset, fair value	11,523	5,240
Level 1
Asset:
Asset, fair value	138,670	136,622
Level 1 | Money Market Fund
Asset:
Asset, fair value	116,456	88,266
Level 1 | U.S. Treasury Shares
Asset:
Asset, fair value	22,214	48,356
Level 2
Asset:
Asset, fair value	281,344	337,260
Level 2 | Corporate Bonds
Asset:
Asset, fair value	95,771	179,864
Level 2 | Commercial Paper
Asset:
Asset, fair value	126,832	106,650
Level 2 | Agency Securities
Asset:
Asset, fair value	6,748	20,782
Level 2 | Repo Securities
Asset:
Asset, fair value	9,000	10,000
Level 2 | Asset Backed Securities
Asset:
Asset, fair value	9,895	8,599
Level 2 | Certificates of Deposit
Asset:
Asset, fair value	21,575	6,125
Level 2 | Non-US Government
Asset:
Asset, fair value	$ 11,523	$ 5,240

X

- Definition

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

+ References

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)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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