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Financial instruments - Fair value liabilities, Narrative (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Disclosure of financial assets [line items]    
Financial assets $ 846,062 $ 930,426
Financial assets measured at fair value | Derivative instruments | Cash flow hedges    
Disclosure of financial assets [line items]    
Financial assets $ 327 $ 0