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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
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Mar. 31, 2019 | Mar. 31, 2018
Basis of Presentation [Line Items] |  |
Share-based compensation expense | $ 5,347 | $ 4,555
Research and development expenses |  |
Basis of Presentation [Line Items] |  |
Share-based compensation expense | 2,127 | 1,767
General and administrative expenses |  |
Basis of Presentation [Line Items] |  |
Share-based compensation expense | $ 3,220 | $ 2,788

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- Definition The amount of expense, net of income tax, recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References No definition available. + Details
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