Shareholders' Equity - Summary of Shares of Common Stock Reserved for Future Issuance under Plans (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Stock Number Of Shares Par Value And Other Disclosures [Abstract]
Stock options outstanding	1,912,161	1,846,768	1,778,697
Restricted stock units outstanding	116,968
Stock options available for future grant	1,189,256
Common stock reserved for future issuance	3,218,385