Stock-Based Compensation - Compensation expense (Details) - USD ($) $ in Thousands |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2019 |
Jun. 30, 2018 |
Jun. 30, 2019 |
Jun. 30, 2018 |
|
| Stock-based compensation expense | ||||
| Stock-based compensation expense | $ 10,604 | $ 5,902 | $ 20,082 | $ 8,515 |
| Unrecognized compensation cost | 89,500 | $ 89,500 | ||
| Unrecognized compensation cost expected to be recognized over a weighted average period | 2 years 4 months 24 days | |||
| Research and development expenses | ||||
| Stock-based compensation expense | ||||
| Stock-based compensation expense | 2,612 | 719 | $ 4,603 | 984 |
| General and administrative expenses | ||||
| Stock-based compensation expense | ||||
| Stock-based compensation expense | $ 7,992 | $ 5,183 | $ 15,479 | $ 7,531 |