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, 2018 |
Sep. 30, 2017 |
Sep. 30, 2018 |
Sep. 30, 2017 |
|
| Basis of Presentation [Line Items] | ||||
| Share-based compensation expense | $ 5,470 | $ 5,248 | $ 14,837 | $ 15,917 |
| Research and development expenses | ||||
| Basis of Presentation [Line Items] | ||||
| Share-based compensation expense | 2,257 | 2,394 | 6,120 | 8,260 |
| General and administrative expenses | ||||
| Basis of Presentation [Line Items] | ||||
| Share-based compensation expense | $ 3,213 | $ 2,854 | $ 8,717 | $ 7,657 |