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Significant Accounting Policies - Estimated Fair Value of Assets and Liability (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Asset:
Asset, fair value	$ 493,278
Money Market Fund
Asset:
Asset, fair value	81,714	$ 10,078
U.S. Treasury Shares
Asset:
Asset, fair value	59,959
Corporate Bonds
Asset:
Asset, fair value	200,704
Commercial Paper
Asset:
Asset, fair value	110,858
Agency Securities
Asset:
Asset, fair value	14,027
Repo Securities
Asset:
Asset, fair value	11,000
Asset Backed Securities
Asset:
Asset, fair value	8,887
Certificate of Deposit
Asset:
Asset, fair value	6,129
Level 1
Asset:
Asset, fair value	141,673
Level 1 | Money Market Fund
Asset:
Asset, fair value	81,714	$ 10,078
Level 1 | U.S. Treasury Shares
Asset:
Asset, fair value	59,959
Level 2
Asset:
Asset, fair value	351,605
Level 2 | Corporate Bonds
Asset:
Asset, fair value	200,704
Level 2 | Commercial Paper
Asset:
Asset, fair value	110,858
Level 2 | Agency Securities
Asset:
Asset, fair value	14,027
Level 2 | Repo Securities
Asset:
Asset, fair value	11,000
Level 2 | Asset Backed Securities
Asset:
Asset, fair value	8,887
Level 2 | Certificate of Deposit
Asset:
Asset, fair value	$ 6,129

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=117815213&loc=d3e19207-110258

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