Stockholders' equity - Schedule of Stock Option Transactions and Number of Stock Options Outstanding (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options outstanding, number of shares, beginning balance	824,604
Options granted, number of shares	287,750
Options exercised, number of shares	(32,113)
Options forfeited, number of shares	0
Options expired, number of shares	(12,442)
Options outstanding, number of shares, ending balance	1,067,799	824,604
Exercisable, number of shares, ending balance	509,192
Options outstanding, weighted average exercise price, beginning balance	$ 6.79
Options granted, weighted average exercise price	11.42
Options exercised, weighted average exercise price	2.98
Options forfeited, weighted average exercise price	0
Options expired, weighted average exercise price	8.34
Options outstanding, weighted average exercise price, ending balance	9.24	$ 6.79
Exercisable, weighted average exercise price, ending balance	$ 7.60
Options outstanding, weighted average remaining contractual life (years), beginning balance	7 years 6 months 29 days	7 years 1 month 28 days
Exercisable, weighted average remaining contractual life (years), ending balance	5 years 10 months 28 days
Options outstanding, aggregate intrinsic value, beginning balance	$ 789
Options granted,aggregate intrinsic value	0
Options exercised,aggregate intrinsic value	0
Options forfeited,aggregate intrinsic value	0
Options expired,aggregate intrinsic value	0
Options outstanding, aggregate intrinsic value, ending balance	425	$ 789
Exercisable, aggregate intrinsic value, ending balance	$ 379