Common stock and preferred stock - Summary of Future Issuance of Common Stock Shares (Detail) - shares shares in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Capitalization [Line Items]
Common shares reserved for future issuance	5,433	4,472
Options to Purchase Common Stock [Member]
Schedule Of Capitalization [Line Items]
Common shares reserved for future issuance	3,735	3,532
Restricted Stock Units [Member]
Schedule Of Capitalization [Line Items]
Common shares reserved for future issuance	263	148
2013 Employee Stock Purchase Plan [Member]
Schedule Of Capitalization [Line Items]
Common shares reserved for future issuance	209	227
2013 Stock Option and Incentive Plan [Member]
Schedule Of Capitalization [Line Items]
Common shares reserved for future issuance	1,226	565