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Fair Value Measurements and Available for Sale Investments - Assets Measured at Fair Value on Recurring Basis (Details) - Recurring Basis - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | $ 367,344 | $ 82,125
U.S. Treasury and agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 53,772 | 91,717
Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 5,580 | 37,411
Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 53,897 | 156,277
Quoted Market Prices for Identical Assets (Level 1) | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 367,344 | 82,125
Quoted Market Prices for Identical Assets (Level 1) | U.S. Treasury and agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 53,772 | 91,717
Quoted Market Prices for Identical Assets (Level 1) | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 0 | 0
Quoted Market Prices for Identical Assets (Level 1) | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 0 | 0
Significant Other Observable Inputs (Level 2) | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 0 | 0
Significant Other Observable Inputs (Level 2) | U.S. Treasury and agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 0 | 0
Significant Other Observable Inputs (Level 2) | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 5,580 | 37,411
Significant Other Observable Inputs (Level 2) | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 53,897 | 156,277
Significant Unobservable Inputs (Level 3) | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 0 | 0
Significant Unobservable Inputs (Level 3) | U.S. Treasury and agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 0 | 0
Significant Unobservable Inputs (Level 3) | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | 0 | 0
Significant Unobservable Inputs (Level 3) | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value on recurring basis | $ 0 | $ 0

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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) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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