Cash and cash equivalents - Schedule of Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Abstract]
Cash	$ 12,583	$ 3,726
Cash equivalents	39,449	28,575
Cash and cash equivalents	$ 52,032	$ 32,301	$ 39,526	$ 14,906