Stockholders' Equity - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Dec. 31, 2021	Dec. 31, 2020	Jul. 15, 2015
Class Of Stock [Line Items]
Total common stock reserved for future issuance	17,205,249	16,386,314
Series A Warrant [Member]
Class Of Stock [Line Items]
Total common stock reserved for future issuance	7,790,879	7,802,241
Series B Warrant [Member]
Class Of Stock [Line Items]
Total common stock reserved for future issuance	2,524,732	2,574,727
Equity Incentive Plans [Member]
Class Of Stock [Line Items]
Total common stock reserved for future issuance	1,207,827	1,117,796
Awards outstanding	4,937,189	4,146,928
2015 Employee Stock Purchase Plan [Member]
Class Of Stock [Line Items]
Total common stock reserved for future issuance	17,500	17,500	17,500
Warrants to Purchase Common Stock [Member]
Class Of Stock [Line Items]
Outstanding warrants	727,122	727,122