Net loss per share - Schedule of Antidilutive Securities Excluded from Computation of Basic and Diluted Net Loss Per Share (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from the computation of basic and diluted net loss per share	870,320	824,721	870,320	8,247,212
Outstanding stock options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from the computation of basic and diluted net loss per share	858,957	813,358	858,957	813,358
Common stock warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from the computation of basic and diluted net loss per share	11,363	11,363	11,363	11,363