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Financial instruments - Fair value liabilities, Narrative (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Disclosure of financial assets [line items]    
Financial assets $ 930,426,000 $ 745,572,000
Financial assets measured at fair value | Derivative instruments | Cash flow hedges    
Disclosure of financial assets [line items]    
Financial assets $ 0 $ 7,024,000