Summary of significant accounting policies - Reconciliation of Shares Used to Determine Net Income per Share (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share Disclosure [Line Items]
Net income (loss)	$ 1,082	$ (108)	$ 5,786	$ 360
Weighted average number of shares outstanding - basic	32,639,429	32,031,426	32,487,230	28,179,853
Weighted average number of shares outstanding - diluted	33,856,045	32,031,426	34,071,982	29,839,975
Warrants
Earnings Per Share Disclosure [Line Items]
Shares underlying	176,775	0	332,269	196,522
Options
Earnings Per Share Disclosure [Line Items]
Shares underlying	1,039,841	0	1,252,483	1,463,600