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Income taxes (Schedule of Components of Deferred Tax Liabilities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Income Taxes [abstract]    
Property, plant and equipment $ 900 $ 972
Deferred tax liabilities 900 972
Alternative minimum tax credits 626
Other 28 138
Total deferred tax assets 654 138
Net deferred tax liabilities $ 246 $ 834