Condensed Balance Sheets (Parenthetical) (Unaudited) - USD ($) $ in Thousands |
Mar. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Statement Of Financial Position [Abstract] | ||
| Convertible preferred stock, par value | $ 0.0001 | $ 0.0001 |
| Convertible preferred stock, shares authorized | 103,283,818 | 91,739,149 |
| Convertible preferred stock, shares issued | 31,750,297 | 28,159,724 |
| Convertible preferred stock, shares outstanding | 31,750,297 | 28,159,724 |
| Convertible preferred stock, liquidation preference | $ 245,825 | $ 190,825 |
| Common stock, par value | $ 0.0001 | $ 0.0001 |
| Common stock, shares authorized | 140,000,000 | 122,000,000 |
| Common stock, shares issued | 5,230,976 | 4,830,389 |
| Common stock, shares outstanding | 5,230,976 | 4,830,389 |