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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Investments - Valuation of Money Market Funds (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	$ 544,120
Level 1
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost of held-to-maturity securities	4,600
Fair market value of held-to-maturity securities	4,600
U.S. government bonds
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	54,475
U.S. government bonds \| Level 1
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	54,475
U.S. government bonds \| Level 2
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	0
U.S. government bonds \| Fair Value, Recurring
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond		$ 6,300
U.S. government bonds \| Fair Value, Recurring \| Level 2
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond		6,348
Money market funds
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	161,837
Money market funds \| Level 1
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	161,837
Money market funds \| Level 2
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	$ 0
Money market funds \| Fair Value, Recurring \| Level 1
Debt and Equity Securities, FV-NI [Line Items]
Cash equivalent money market funds		384,474
Money market funds \| Fair Value, Recurring \| Level 1 \| Revision of Prior Period, Adjustment
Debt and Equity Securities, FV-NI [Line Items]
Cash equivalent money market funds		$ 286,700