Common Stock - Summary of Reserved Shares of Common Stock for Future Issuances (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016
Class Of Stock [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance	6,164,196	5,998,840
Stock Compensation Plan [Member]
Class Of Stock [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance		1,044,113
Options to purchase common stock [Member]
Class Of Stock [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance	3,645,456	4,452,700
Restricted stock units [Member]
Class Of Stock [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance	2,518,740	502,027