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Summary of significant accounting policies - Right-of-Use Assets (Details) - USD ($)
$ in Thousands
 12 Months Ended
Jan. 01, 2019
Dec. 31, 2018
Right-of-Use Assets Activity [Roll Forward]    
Opening net book amount $ 0  
Initial recognition of right-of-use assets 204,937 $ 204,937
Reclassifications from Trade and other receivables, net 0  
Closing net book amount 204,937 0
Lease Liabilities Activity [Roll Forward]    
Lease liabilities 0  
Initial recognition of lease liabilities (204,937)  
Reclassifications to lease liabilities from trade and other receivables, net 26,794  
Lease liabilities $ (178,143) $ 0