Fair Value Disclosure - Schedule of Financial Assets Measured at Fair Value (Details) - USD ($) |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Assets: | ||
| Cash and cash equivalents | $ 0 | |
| Fair Value | $ 42,146,000 | |
| Total assets measured and recorded at fair value | 197,710,000 | |
| Level 1 | ||
| Assets: | ||
| Total assets measured and recorded at fair value | 155,564,000 | |
| Level 2 | ||
| Assets: | ||
| Total assets measured and recorded at fair value | 42,146,000 | |
| Cash | ||
| Assets: | ||
| Cash and cash equivalents | 147,221,000 | |
| Cash | Level 1 | ||
| Assets: | ||
| Cash and cash equivalents | 147,221,000 | |
| Money Market Fund | ||
| Assets: | ||
| Cash and cash equivalents | 8,343,000 | |
| Money Market Fund | Level 1 | ||
| Assets: | ||
| Cash and cash equivalents | 8,343,000 | |
| Corporate Bonds and Commercial Paper | ||
| Assets: | ||
| Fair Value | 37,151,000 | |
| Corporate Bonds and Commercial Paper | Level 2 | ||
| Assets: | ||
| Fair Value | 37,151,000 | |
| U.S. Government Securities | ||
| Assets: | ||
| Fair Value | 4,995,000 | |
| U.S. Government Securities | Level 2 | ||
| Assets: | ||
| Fair Value | $ 4,995,000 |