Common stock - Public Offering of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Mar. 27, 2018	Mar. 31, 2017
Proceeds from the issuance of common stock		$ 10,000
Common Stock | Pub
Shares issued	2,146,667
Price per share (in dollars per share)	$ 7.50
Proceeds from the issuance of common stock	$ 14,800
Common Stock | Over-Allotment Option
Shares issued	280,000