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Income taxes - Narratives (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Operating Loss Carryforwards [Line Items]    
Uncertain tax positions $ 1.6 $ 1.5
Operating loss carryforwards $ 48.5  
Bermuda    
Operating Loss Carryforwards [Line Items]    
Effective tax rate 0.00%  
United States    
Operating Loss Carryforwards [Line Items]    
Effective tax rate 21.00%  
United Kingdom    
Operating Loss Carryforwards [Line Items]    
Effective tax rate 19.00%