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Commitments and Contingencies - Concentrations of credit risk (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Concentration Risk [Line Items]      
Concentration risk percentage 10.00%    
Customer Concentration Risk | Gross Written Premiums | Aon Corporation      
Concentration Risk [Line Items]      
Concentration risk percentage 12.20% 11.40% 11.30%
Customer Concentration Risk | Gross Written Premiums | Marsh And McLennan Companies      
Concentration Risk [Line Items]      
Concentration risk percentage 9.60% 9.30% 10.70%