Share-based compensation - Summary of Status of Company's Stock Option Plan (Detail) - Stock Option Plan - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Shares
Outstanding at beginning of period	3,069,440	3,214,398
-Exercised	(5,000)	(1,545,161)
-Granted	20,000	1,400,203
-Cancelled	(135,592)
Outstanding at end of period	2,948,848	3,069,440
Weighted Average Exercise Price
Outstanding at beginning of period	$ 4.28	$ 2.68
-Exercised	3.43	2.27
-Granted	5.81	5.72
-Cancelled	6.26
Outstanding at end of period	$ 4.2	$ 4.28