Share-based compensation - Summary of Status of Company's Stock Option Plan (Detail) - Stock Option Plan - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Shares
Outstanding at beginning of period	2,633,589	3,280,874
-Cancelled		(132,500)
-Exercised	(140,125)	(679,291)
-Granted	50,000	164,506
Outstanding at end of period	2,543,464	2,633,589
Weighted Average Exercise Price
Outstanding at beginning of period	$ 2.06	$ 1.98
-Cancelled		3.72
-Exercised	1.29	1.65
-Granted	3.43	3.28
Outstanding at end of period	$ 2.13	$ 2.06