Share-based compensation - Summary of Status of Company's Stock Option Plan (Detail) - Stock Option Plan - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Shares
Outstanding at beginning of period	2,633,589	3,280,874
-Cancelled		(132,500)
-Exercised	(530,425)	(679,291)
-Granted	1,170,534	164,506
Outstanding at end of period	3,273,698	2,633,589
Weighted Average Exercise Price
Outstanding at beginning of period	$ 2.06	$ 1.98
-Cancelled		3.72
-Exercised	2.38	1.65
-Granted	3.95	3.28
Outstanding at end of period	$ 2.68	$ 2.06