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Fair Value Measurements and Fair Value of Financial Instruments - Unrealized Gain (Loss) (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Amortized Cost Basis | $ 505,518 | $ 431,808
Unrealized Gains | 12 | 57
Unrealized Losses | (20) | (2)
Fair Value | 505,510 | 431,863
Money market funds |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Amortized Cost Basis | 330,308 | 333,585
Unrealized Gains | 0 | 0
Unrealized Losses | 0 | 0
Fair Value | 330,308 | 333,585
U.S. Treasury securities |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Amortized Cost Basis | 175,210 | 98,223
Unrealized Gains | 12 | 57
Unrealized Losses | (20) | (2)
Fair Value | $ 175,202 | $ 98,278

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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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 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454 + Details
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