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Commitments and contingencies - Right-of-use lease asset balances (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Commitments and contingencies    
Carrying amount, January 1, 2020 $ 82,794  
ROU assets acquired (Note 4) 155,662  
Additions to leased assets 53,527  
Depreciation charges (22,553) $ (10,270)
Transferred to assets held for sale 2,262  
Carrying amount, December 31, 2020 $ 267,168 $ 82,794