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Finance costs (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Borrowing costs [abstract]    
Finance costs before capitalized interest $ 182,841,000 $ 127,282,000
Less capitalized interest (18,004,000) (2,856,000)
Finance costs 164,837,000 124,426,000
Disclosure of detailed information about borrowings [line items]    
Long-term debt 2,363,372,000 $ 1,768,853,000
Unsecured notes due March 2020    
Disclosure of detailed information about borrowings [line items]    
Make-whole interest charge on borrowings 15,400,000  
Long-term debt $ 250,000,000