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, 2017	Dec. 31, 2016
Shares
Outstanding at beginning of period	3,214,398	2,633,589
-Exercised	(159,361)	(589,725)
-Granted		1,170,534
Outstanding at end of period	3,055,037	3,214,398
Weighted Average Exercise Price
Outstanding at beginning of period	$ 2.68	$ 2.06
-Exercised	2.23	2.43
-Granted		3.95
Outstanding at end of period	$ 2.7	$ 2.68