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Fair Value of Financial Assets - Summary of Financial Assets Measured at Fair Value on a Recurring Basis and Level of the Fair Value Hierarchy (Detail) - Fair value on a recurring basis [Member] - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investments:
Total	$ 677,561	$ 355,520
Investments [Member]
Investments:
Total investments	230,415	313,862
Investments [Member] | US treasury bills [Member]
Investments:
Total investments	33,026	232,604
Investments [Member] | US agency securities [Member]
Investments:
Total investments	197,389	81,258
Level 1 [Member]
Investments:
Total	447,146	41,658
Level 2 [Member]
Investments:
Total	230,415	313,862
Level 2 [Member] | Investments [Member]
Investments:
Total investments	230,415	313,862
Level 2 [Member] | Investments [Member] | US treasury bills [Member]
Investments:
Total investments	33,026	232,604
Level 2 [Member] | Investments [Member] | US agency securities [Member]
Investments:
Total investments	197,389	81,258
Cash equivalents [Member] | Money market funds [Member]
Cash equivalents:
Assets, fair value	447,146	41,658
Cash equivalents [Member] | Level 1 [Member] | Money market funds [Member]
Cash equivalents:
Assets, fair value	$ 447,146	$ 41,658

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Cash and cash equivalents fair value disclosure.

+ References

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- 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)
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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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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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