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Short-term Investments - Additional Information (Detail) - US Treasury Securities - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Summary Of Investment Holdings [Line Items]
Available-for-sale securities, amortized cost	$ 108,800,000	$ 466,600,000
Available-for-sale securities, fair value	108,900,000	466,200,000
Available-for-sale securities, other-than-temporary impairments	$ 0	$ 0
Minimum
Summary Of Investment Holdings [Line Items]
Available-for-sale securities maturity range	3 months
Maximum
Summary Of Investment Holdings [Line Items]
Available-for-sale securities maturity range	1 year

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- Definition

Amortized cost 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
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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).

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+ References

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Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings and other comprehensive loss (OCI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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