Share-based compensation - Summary of Status of Company's Stock Option Plan (Detail) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Shares
-Exercised	(984,861)
Stock Option Plan
Shares
Outstanding at beginning of period	3,214,398	2,633,589
-Exercised	(984,861)	(589,725)
-Granted		1,170,534
Outstanding at end of period	2,229,537	3,214,398
Weighted Average Exercise Price
Outstanding at beginning of period	$ 2.68	$ 2.06
-Exercised	3.07	2.43
-Granted		3.95
Outstanding at end of period	$ 2.5	$ 2.68