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, 2015	Sep. 30, 2014	Dec. 31, 2014
Shares
Outstanding at beginning of period	3,280,874	2,517,911	2,517,911
Exercised	(452,841)		(292,537)
Granted	164,506	10,000	1,055,500
Outstanding at end of period	2,992,539		3,280,874
Weighted Average Exercise Price
Outstanding at beginning of period	$ 1.98	$ 1.33	$ 1.33
Exercised	1.59		1.03
Granted	3.28		3.28
Outstanding at end of period	$ 2.11		$ 1.98