Stockholders' Equity - Schedule of Shares of Common Stock Reserved for Future Issuance (Detail) - shares	Jun. 30, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Total common stock reserved for future issuance	11,210,093	9,295,176
Options to Purchase Common Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	5,146,089	4,599,252
Available Future Grant Year [Member] | Options to Purchase Common Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	4,365,896	3,403,597
2014 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	1,698,108	1,292,327