Cash and cash equivalents - Schedule of Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Abstract]
Cash	$ 23,538	$ 3,726
Cash equivalents	23,567	28,575
Cash and cash equivalents	$ 47,105	$ 32,301	$ 86,225	$ 39,526