Stock-Based Compensation - Granted to non-employees, prior to the adoption of ASU 2018-07 (Details) - Non-employees - ASU 2018-07 - $ / shares |
12 Months Ended | |
|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Minimum | ||
| Fair value of stock awards granted prior to the adoption of ASU 2018-07 | ||
| Risk-free interest rate | 2.02% | 2.35% |
| Expected volatility | 74.00% | 81.00% |
| Expected term (in years) | 6 years | 8 years |
| Fair value of common stock | $ 0.19 | $ 0.18 |
| Maximum | ||
| Fair value of stock awards granted prior to the adoption of ASU 2018-07 | ||
| Risk-free interest rate | 2.31% | 2.45% |
| Expected volatility | 85.00% | 85.00% |
| Expected term (in years) | 10 years | 10 years |
| Fair value of common stock | $ 6.28 | $ 0.19 |