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Tax matters - Significant components of deferred tax assets and liabilities (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Deferred tax assets $ 14,589 $ 5,273
Deferred tax liabilities 77,379 65,142
Net Total Deferred Tax Asset / (Liability) (62,790) (59,869)
Total $ 483,459 465,663
Number of years open to review 4 years  
Unused tax losses    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Total $ 396,119 391,997
Unused tax credits    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Total 7,963 8,028
Unrecognised deductible temporary differences    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Total 79,377 65,638
Hedging instruments    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Net Total Deferred Tax Asset / (Liability) 1,010 1,239
Other    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Net Total Deferred Tax Asset / (Liability) $ (678) $ (6,028)