Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail)	Mar. 31, 2019 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	23,897,681
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	8,219,805
Common Stock Warrants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	6,682,708
Common Stock Options and Awards Outstanding [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	5,671,937
Shares Available Under the 2015 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	2,178,136
Shares Available Under the 2015 ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	1,145,095