Stockholders' equity - Schedule of Stock Option Transactions and Number of Stock Options Outstanding (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 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	271,750
Options exercised, number of shares	(28,020)
Options forfeited, number of shares	0
Options expired, number of shares	(11,442)
Options outstanding, number of shares, ending balance	1,056,892	824,604
Exercisable, number of shares, ending balance	436,129
Options outstanding, weighted average exercise price, beginning balance	$ 6.79
Options granted, weighted average exercise price	$ 11.70
Options exercised, weighted average exercise price	$ 2.57
Options forfeited, weighted average exercise price	$ 0
Options expired, weighted average exercise price	$ 8.25
Options outstanding, weighted average exercise price, ending balance	$ 9.26	$ 6.79
Exercisable, weighted average exercise price, ending balance	$ 7.23
Options outstanding, weighted average remaining contractual life (years), beginning balance	7 years 10 months 10 days	7 years 1 month 28 days
Exercisable, weighted average remaining contractual life (years), ending balance	5 years 10 months 17 days
Options outstanding, aggregate intrinsic value, beginning balance	$ 789
Options outstanding, aggregate intrinsic value, ending balance	922	789
Exercisable, aggregate intrinsic value, ending balance	$ 767