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Basis of Presentation and Summary of Significant Accounting Policies - Accounting for Share-Based Compensation (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Basis of Presentation [Line Items] |  |  |  |
Share-based compensation expense | $ 9,754 | $ 7,740 | $ 28,375 | $ 20,752
SBC - Research and development expenses |  |  |  |
Basis of Presentation [Line Items] |  |  |  |
Share-based compensation expense | 4,480 | 3,094 | 12,975 | 8,510
SBC - General and administrative expenses |  |  |  |
Basis of Presentation [Line Items] |  |  |  |
Share-based compensation expense | $ 5,274 | $ 4,646 | $ 15,400 | $ 12,242

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