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Fair Value Measurement - Financial Instruments at Fair Value (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Assets: |  |
Short-term investments | $ 665,567 | $ 381,074
Fair Value on Recurring Basis |  |
Assets: |  |
Total assets measured and recorded at fair value | 1,653,994 | 837,722
Fair Value on Recurring Basis | Agency Securities |  |
Assets: |  |
Short-term investments |  | 10,006
Long-term investments | 39,031 | 15,005
Fair Value on Recurring Basis | US Treasury Securities |  |
Assets: |  |
Short-term investments |  | 44,286
Fair Value on Recurring Basis | Corporate Bond Securities |  |
Assets: |  |
Short-term investments | 461,397 | 319,293
Long-term investments | 484,597 | 295,478
Fair Value on Recurring Basis | Commercial paper |  |
Assets: |  |
Short-term investments | 204,170 | 7,489
Fair Value on Recurring Basis | Money Market Funds |  |
Assets: |  |
Cash equivalents | 464,799 | 146,165
Fair Value on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets: |  |
Total assets measured and recorded at fair value | 464,799 | 190,451
Fair Value on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Agency Securities |  |
Assets: |  |
Short-term investments |  | 0
Long-term investments | 0 | 0
Fair Value on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury Securities |  |
Assets: |  |
Short-term investments |  | 44,286
Fair Value on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate Bond Securities |  |
Assets: |  |
Short-term investments | 0 | 0
Long-term investments | 0 | 0
Fair Value on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial paper |  |
Assets: |  |
Short-term investments | 0 | 0
Fair Value on Recurring Basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money Market Funds |  |
Assets: |  |
Cash equivalents | 464,799 | 146,165
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) |  |
Assets: |  |
Total assets measured and recorded at fair value | 1,189,195 | 647,271
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) | Agency Securities |  |
Assets: |  |
Short-term investments |  | 10,006
Long-term investments | 39,031 | 15,005
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) | US Treasury Securities |  |
Assets: |  |
Short-term investments |  | 0
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) | Corporate Bond Securities |  |
Assets: |  |
Short-term investments | 461,397 | 319,293
Long-term investments | 484,597 | 295,478
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) | Commercial paper |  |
Assets: |  |
Short-term investments | 204,170 | 7,489
Fair Value on Recurring Basis | Significant Other Observable Inputs (Level 2) | Money Market Funds |  |
Assets: |  |
Cash equivalents | $ 0 | $ 0

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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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