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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, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Basis of Presentation [Line Items] |  |  |  |
Share-based compensation expense | $ 6,297 | $ 5,470 | $ 18,215 | $ 14,837
Research and development expenses |  |  |  |
Basis of Presentation [Line Items] |  |  |  |
Share-based compensation expense | 2,481 | 2,257 | 7,136 | 6,120
General and administrative expenses |  |  |  |
Basis of Presentation [Line Items] |  |  |  |
Share-based compensation expense | $ 3,816 | $ 3,213 | $ 11,079 | $ 8,717

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