Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail)	Sep. 30, 2017 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	24,161,061
Class X Preferred Stock (if-Converted to Common Stock) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	11,429,760
Common Stock Warrants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	6,642,332
Common Stock Options and Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	4,745,715
Shares Available Under the 2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	729,139
Shares Available Under the 2015 ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	614,115