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Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Financial liabilities: |  |
Success payment liabilities | $ 25,116 | $ 5,773
Fair Value, Measurements, Recurring |  |
Financial assets: |  |
Financial assets, fair value | 916,899 | 671,410
Financial liabilities: |  |
Success payment liabilities | 25,116 | 5,773
Total financial liabilities | 25,116 | 5,773
Fair Value, Measurements, Recurring | Money Market Funds [Member] |  |
Financial assets: |  |
Financial assets, fair value | 331,349 | 50,513
Fair Value, Measurements, Recurring | U.S. Treasury Securities [Member] |  |
Financial assets: |  |
Financial assets, fair value | 249,761 | 202,701
Fair Value, Measurements, Recurring | U.S. government agency securities |  |
Financial assets: |  |
Financial assets, fair value | 103,961 | 205,764
Fair Value, Measurements, Recurring | Corporate debt securities |  |
Financial assets: |  |
Financial assets, fair value | 230,704 | 211,109
Fair Value, Measurements, Recurring | Equity warrant investment |  |
Financial assets: |  |
Financial assets, fair value | 1,124 | 1,323
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |
Financial assets: |  |
Financial assets, fair value | 331,349 | 50,513
Financial liabilities: |  |
Success payment liabilities | 0 | 0
Total financial liabilities | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Money Market Funds [Member] |  |
Financial assets: |  |
Financial assets, fair value | 331,349 | 50,513
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | U.S. Treasury Securities [Member] |  |
Financial assets: |  |
Financial assets, fair value | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | U.S. government agency securities |  |
Financial assets: |  |
Financial assets, fair value | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Corporate debt securities |  |
Financial assets: |  |
Financial assets, fair value | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Equity warrant investment |  |
Financial assets: |  |
Financial assets, fair value | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |
Financial assets: |  |
Financial assets, fair value | 584,426 | 619,574
Financial liabilities: |  |
Success payment liabilities | 0 | 0
Total financial liabilities | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Money Market Funds [Member] |  |
Financial assets: |  |
Financial assets, fair value | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | U.S. Treasury Securities [Member] |  |
Financial assets: |  |
Financial assets, fair value | 249,761 | 202,701
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | U.S. government agency securities |  |
Financial assets: |  |
Financial assets, fair value | 103,961 | 205,764
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Corporate debt securities |  |
Financial assets: |  |
Financial assets, fair value | 230,704 | 211,109
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Equity warrant investment |  |
Financial assets: |  |
Financial assets, fair value | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |
Financial assets: |  |
Financial assets, fair value | 1,124 | 1,323
Financial liabilities: |  |
Success payment liabilities | 25,116 | 5,773
Total financial liabilities | 25,116 | 5,773
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Money Market Funds [Member] |  |
Financial assets: |  |
Financial assets, fair value | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | U.S. Treasury Securities [Member] |  |
Financial assets: |  |
Financial assets, fair value | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | U.S. government agency securities |  |
Financial assets: |  |
Financial assets, fair value | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Corporate debt securities |  |
Financial assets: |  |
Financial assets, fair value | 0 | 0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Equity warrant investment |  |
Financial assets: |  |
Financial assets, fair value | $ 1,124 | $ 1,323

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