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Financial Risk Management - Credit Risk - Reinsurance contract assets (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Financial Risk Management    
Capital and surplus requirement $ 500.0  
Funds available for collateralization 1,000.0  
Reinsurance contract assets held $ 10,682.6 $ 10,887.7
A.M. Best rating A- or higher    
Financial Risk Management    
Risk exposure percentage 88.00%  
Pools and associations    
Financial Risk Management    
Risk exposure percentage 12.00%