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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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+ References

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- Definition

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+ 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
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Fair value of financial and nonfinancial obligations.

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