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Significant accounting policies - Application of new and revised accounting standards (Details) - USD ($)
$ in Thousands
 12 Months Ended
Jan. 01, 2019
Dec. 31, 2019
Dec. 31, 2018
Disclosure of expected impact of initial application of new standards or interpretations [line items]      
Non-current assets   $ 3,964,094 $ 3,356,923
Retained earnings   1,039,819 1,145,476
Weighted average lessee's incremental borrowing rate applied to lease liabilities recognised at date of initial application of IFRS 16 4.40%    
Recognition exception for:      
Short-term leases   $ (500)  
Lease liabilities at January 1, 2019 $ 452,882    
IFRS 16, Leases      
Disclosure of expected impact of initial application of new standards or interpretations [line items]      
Non-current assets 411,000    
Total liabilities 453,000    
Retained earnings 42,000    
Retained earnings, net of tax $ 38,000    
Weighted average lessee's incremental borrowing rate applied to lease liabilities recognised at date of initial application of IFRS 16 4.40%    
Gross lease liabilities     $ 427,289
Recognition exception for:      
Short-term leases $ (777)    
Leases of low-value assets (8)    
Extension and termination options reasonably certain to be exercised 75,753    
Scope changes due to IFRS 16 18,880    
Other 594    
Lease liabilities at January 1, 2019 $ 358,440