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Investments - Summary of Gross Unrealized Losses and Fair Value of Available-For-Sale Investment Securities in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	$ 169,353	$ 220,834
Less Than 12 Months, Unrealized Losses	(43)	(51)
Total, Estimated Fair Value	169,353	220,834
Total, Unrealized Losses	(43)	(51)
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	129,631	148,397
Less Than 12 Months, Unrealized Losses	(27)	(13)
Total, Estimated Fair Value	129,631	148,397
Total, Unrealized Losses	(27)	(13)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	6,252	11,334
Less Than 12 Months, Unrealized Losses		(8)
Total, Estimated Fair Value	6,252	11,334
Total, Unrealized Losses		(8)
US Government Sponsored Enterprises Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	10,004	61,103
Less Than 12 Months, Unrealized Losses	(5)	(30)
Total, Estimated Fair Value	10,004	61,103
Total, Unrealized Losses	(5)	$ (30)
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Estimated Fair Value	23,466
Less Than 12 Months, Unrealized Losses	(11)
Total, Estimated Fair Value	23,466
Total, Unrealized Losses	$ (11)

X

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27337-111563

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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