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Income taxes - Summary of change in net deferred tax position (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Net deferred tax asset (liability)    
Beginning balance $ 412,717 $ 429,929
Deferred tax recovery (9,441) (16,579)
Deferred tax recovery in the consolidated statement of OCI (563) (633)
Ending balance $ 402,713 $ 412,717