Shareholders' equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 26,014
Net income	$ 1,210	$ 867	1,890	$ 1,810
Share-based compensation expense			337
Issuance of deferred stock units, net of relinquishments			(265)
Issuance of shares from exercise of stock options, net of tax benefit			108
Foreign currency translation adjustment	(10)	$ 5	(11)	$ 7
Dividends declared and paid on common stock			(1,336)
Purchase of common stock for treasury			(2,000)
Balance at December 31, 2017	$ 24,737		$ 24,737
Dividends [Abstract]
Dividends declared and paid per common share (in dollars per share)	$ 0.09	$ 0.09	$ 0.18	$ 0.17
Payment of dividend on common stock	$ 663	$ 663	$ 1,336	$ 1,250

Amount of increase to additional paid-in capital (APIC) from recognition of equity-based compensation.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Amount after tax and reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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