Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands |
Mar. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Liabilities: | ||
| Warrant liability | $ 5,647 | $ 2,766 |
| Recurring basis | ||
| Assets: | ||
| Fair value of assets | 21,941 | 21,597 |
| Liabilities: | ||
| Fair value of liabilities | 5,647 | 2,766 |
| Recurring basis | Series D preferred stock warrants | ||
| Liabilities: | ||
| Warrant liability | 1,320 | 992 |
| Recurring basis | Common stock warrants | ||
| Liabilities: | ||
| Warrant liability | 4,327 | 1,774 |
| Recurring basis | Money market funds | ||
| Assets: | ||
| Cash Equivalents | 21,941 | 21,597 |
| Level 1 | Recurring basis | ||
| Assets: | ||
| Fair value of assets | 21,941 | 21,597 |
| Level 1 | Recurring basis | Money market funds | ||
| Assets: | ||
| Cash Equivalents | 21,941 | 21,597 |
| Level 3 | Recurring basis | ||
| Liabilities: | ||
| Fair value of liabilities | 5,647 | 2,766 |
| Level 3 | Recurring basis | Series D preferred stock warrants | ||
| Liabilities: | ||
| Warrant liability | 1,320 | 992 |
| Level 3 | Recurring basis | Common stock warrants | ||
| Liabilities: | ||
| Warrant liability | $ 4,327 | $ 1,774 |