Stockholders' equity - Schedule of Stock Option Transactions and Number of Stock Options Outstanding (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options Outstanding, Number of Shares, beginning balance	865,457
Options granted, Number of Shares	396,960
Options forfeited, Number of Shares	(6,844)
Options Outstanding, Number of Shares, ending balance	1,255,573	865,457
Exercisable, Number of Shares, ending balance	510,584
Options Outstanding, Weighted Average Exercise Price, beginning balance	$ 9.75
Options granted, Weighted Average Exercise Price	8.29
Options forfeited, Weighted Average Exercise Price	12.25
Options Outstanding, Weighted Average Exercise Price, ending balance	9.27	$ 9.75
Exercisable, Weighted Average Exercise Price, ending balance	$ 8.87
Options Outstanding, Weighted Average Remaining Contractual Life (Years), beginning balance	8 years 1 month 17 days	7 years 6 months 22 days
Exercisable, Weighted Average Remaining Contractual Life (Years), ending balance	6 years 6 months 4 days
Options Outstanding, Aggregate Intrinsic Value	$ 999	$ 2,393
Exercisable, Aggregate Intrinsic Value	$ 677