Shareholders' equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 24,109		$ 24,109
Net income	$ 1,813	$ 883			3,623	$ 2,261
Share-based compensation expense					484
Issuance of deferred stock units, net of relinquishments					(23)
Issuance of shares from exercise of stock options, net of tax benefit					294
Reversal of deferred tax asset in connection with stock options forfeited					(110)
Foreign currency translation adjustment	3	$ (3)			10	$ (21)
Dividends declared and paid on common stock					(1,911)
Purchase of common stock for treasury					(358)
Ending balance	$ 26,118				$ 26,118
Dividends [Abstract]
Dividends declared and paid per common share (in dollars per share)	$ 0.09	$ 0.08	$ 0.26	$ 0.24	$ 0.26	$ 0.24
Payment of dividend on common stock	$ 661	$ 595	$ 1,911	$ 1,827	$ 1,911	$ 1,827