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Intangible assets (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of changes in intangible assets other than goodwill [abstract]      
Beginning Balance $ 2,909 $ 427  
Ending Balance 5,425 2,909 $ 427
Gross amounts of assets [member]      
Reconciliation of changes in intangible assets other than goodwill [abstract]      
Beginning Balance 13,883 10,776 10,987
Additions other than through business combinations, intangible assets other than goodwill 3,370 3,111 0
Disposals and retirements, intangible assets other than goodwill (81) (4) (211)
Ending Balance 17,172 13,883 10,776
Accumulated amortization [member]      
Reconciliation of changes in intangible assets other than goodwill [abstract]      
Beginning Balance 10,974 10,349 9,963
Disposals and retirements, intangible assets other than goodwill (65) (4) (210)
Ending Balance 11,747 10,974 10,349
Intangible Assets Carrying Value 5,425 2,909 427
Changes in intangible assets other than goodwill [abstract]      
Amortization expense of the year $ 838 $ 629 $ 596