Net loss per common stock - Schedule of Antidilutive Securities Excluded from Computation of Basic and Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	2,069,167	1,378,352	876,820
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	2,069,167	1,378,352	865,457
Warrants to Purchase Common Stock [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