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Fair Value Measurements and Available for Sale Investments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | $ 165,369 |
U.S. Treasury securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 107,159 |
Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 3,713 |
Agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 15,805 |
Commercial and corporate obligations |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 38,692 |
Fair Value, Measurements, Recurring | U.S. Treasury securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 107,159 | $ 96,434
Fair Value, Measurements, Recurring | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 3,713 | 5,428
Fair Value, Measurements, Recurring | Agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 15,805 | 33,623
Fair Value, Measurements, Recurring | Commercial and corporate obligations |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 38,692 | 122,030
Fair Value, Measurements, Recurring | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at fair value | 204,678 | 162,928
Fair Value, Measurements, Recurring | Mutual funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at fair value | 4,833 | 7,619
Fair Value, Measurements, Recurring | Quoted Market Prices for Identical Assets (Level 1) | U.S. Treasury securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 107,159 | 96,434
Fair Value, Measurements, Recurring | Quoted Market Prices for Identical Assets (Level 1) | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 0 | 0
Fair Value, Measurements, Recurring | Quoted Market Prices for Identical Assets (Level 1) | Agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 0 | 0
Fair Value, Measurements, Recurring | Quoted Market Prices for Identical Assets (Level 1) | Commercial and corporate obligations |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 0 | 0
Fair Value, Measurements, Recurring | Quoted Market Prices for Identical Assets (Level 1) | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at fair value | 204,678 | 162,928
Fair Value, Measurements, Recurring | Quoted Market Prices for Identical Assets (Level 1) | Mutual funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at fair value | 4,833 | 7,619
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 0 | 0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 3,713 | 5,428
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 15,805 | 33,623
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Commercial and corporate obligations |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 38,692 | 122,030
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at fair value | 0 | 0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Mutual funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at fair value | 0 | 0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 0 | 0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Certificates of deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 0 | 0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 0 | 0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Commercial and corporate obligations |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, debt securities | 0 | 0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at fair value | 0 | 0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Mutual funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at fair value | $ 0 | $ 0

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