Stockholders' equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Dec. 14, 2016	Dec. 08, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stockholders Equity [Line Items]
Common Stock, Shares, Issued			42,602,431	42,398,140
Proceeds from Issuance of Common Stock			$ 17	$ 807	$ 51,060
Underwritten Offering [Member]
Stockholders Equity [Line Items]
Common Stock, Shares, Issued		6,428,571
Stock Issued During Period, Shares, New Issues		0.01
Proceeds from Issuance of Common Stock	$ 48,400
Additional Offering Expenses	$ 3,300
Underwriters [Member]
Stockholders Equity [Line Items]
Stock Issued During Period, Shares, New Issues	7,392,856	6,428,571
Purchase Of Additional Common Stock	964,285	964,285
Proceeds from Issuance of Common Stock	$ 51,700
Share Price	$ 7.00