Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Mar. 31, 2019	Dec. 31, 2018	Jul. 15, 2015
Class of Stock [Line Items]
Total common stock reserved for future issuance	16,022,345	13,370,025
Equity Incentive Plans [Member]
Class of Stock [Line Items]
Awards outstanding	13,198,385	10,504,412
Total common stock reserved for future issuance	1,903,372	1,945,025
2015 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	700,000	700,000	700,000
Warrants to Purchase Common Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	147,059	147,059
Outstanding warrant	73,529	73,529