Stockholders' Equity - Schedule of Shares of Common Stock Reserved for Future Issuance (Detail) - shares	Mar. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Total common stock reserved for future issuance	13,149,357	11,154,806
Options to Purchase Common Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	7,346,935	5,019,964
Available Future Grant Year [Member] | Options to Purchase Common Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	3,699,320	4,492,021
2014 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	2,103,102	1,642,821