Shareholders' equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 26,014
Net income	680	$ 943
Share-based compensation expense	161
Issuance of deferred stock units, net of relinquishments	(246)
Issuance of shares from exercise of stock options, net of tax benefit	94
Foreign currency translation adjustment	(1)	$ 2
Dividends declared and paid on common stock	(673)
Purchase of common stock for treasury	(1,562)
Balance at December 31, 2017	$ 24,467
Dividends [Abstract]
Dividends declared and paid per common share (in dollars per share)	$ 0.09	$ 0.08
Payment of dividend on common stock	$ 673	$ 587