Shareholders' equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 24,109
Net income	$ 867	$ 753	1,810	$ 1,378
Share-based compensation expense			296
Issuance of deferred stock units, net of relinquishments			(18)
Issuance of shares from exercise of stock options, net of tax benefit			85
Reversal of deferred tax asset in connection with stock options forfeited			(99)
Foreign currency translation adjustment	5	$ (14)	7	$ (18)
Dividends declared and paid on common stock			(1,250)
Ending balance	$ 24,940		$ 24,940
Dividends [Abstract]
Dividends declared and paid per common share (in dollars per share)	$ 0.09	$ 0.08	$ 0.17	$ 0.16
Payment of dividend on common stock	$ 663	$ 609	$ 1,250	$ 1,232