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Segment reporting - Concentration Risk (Details) - Gross Premiums Written
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenue, Major Customer [Line Items]      
Concentration risk, percentage 100.00% 100.00% 100.00%
Contract Concentration Risk      
Revenue, Major Customer [Line Items]      
Concentration risk, percentage 100.00% 100.00% 100.00%
Contract Concentration Risk | Largest contract      
Revenue, Major Customer [Line Items]      
Concentration risk, percentage   14.40% 15.50%
Contract Concentration Risk | Second largest contract      
Revenue, Major Customer [Line Items]      
Concentration risk, percentage     10.70%
Contract Concentration Risk | Total for contracts contributing greater than 10% each      
Revenue, Major Customer [Line Items]      
Concentration risk, percentage 0.00% 14.40% 26.20%
Contract Concentration Risk | Total for contracts contributing less than 10% each      
Revenue, Major Customer [Line Items]      
Concentration risk, percentage 100.00% 85.60% 73.80%