Fair Value - Schedule of Financial Assets Measured at Fair Value (Details) - USD ($) $ in Thousands |
Jun. 30, 2019 |
Dec. 31, 2018 |
|---|---|---|
| Assets: | ||
| Restricted cash | $ 868 | $ 0 |
| Fair Value | 10,987 | 42,146 |
| Total assets measured and recorded at fair value | 387,364 | 197,710 |
| Level 1 | ||
| Assets: | ||
| Restricted cash | 868 | |
| Total assets measured and recorded at fair value | 376,377 | 155,564 |
| Level 2 | ||
| Assets: | ||
| Total assets measured and recorded at fair value | 10,987 | 42,146 |
| Cash | ||
| Assets: | ||
| Cash and cash equivalents | 335,320 | 147,221 |
| Cash | Level 1 | ||
| Assets: | ||
| Cash and cash equivalents | 335,320 | 147,221 |
| Money Market Fund | ||
| Assets: | ||
| Cash and cash equivalents | 40,189 | 8,343 |
| Money Market Fund | Level 1 | ||
| Assets: | ||
| Cash and cash equivalents | 40,189 | 8,343 |
| Corporate Bonds and Commercial Paper | ||
| Assets: | ||
| Fair Value | 5,992 | 37,151 |
| Corporate Bonds and Commercial Paper | Level 2 | ||
| Assets: | ||
| Fair Value | 5,992 | 37,151 |
| U.S. Government Securities | ||
| Assets: | ||
| Fair Value | 4,995 | 4,995 |
| U.S. Government Securities | Level 2 | ||
| Assets: | ||
| Fair Value | $ 4,995 | $ 4,995 |