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Income taxes - Schedule of Deferred Tax Assets and Liabilities Within the Same Jurisdiction (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]    
Deferred tax assets $ 245 $ 37
Deferred tax liabilities 21,341 34,212
Net deferred tax liabilities $ (21,096) $ (34,175)