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Reinsurance - Summary of Assumed and Ceded Reinsurance on Premiums Written, Premiums Earned and Insurance Losses and Loss Adjustment Expenses (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Premiums written:      
Insurance $ 2,723.5 $ 2,446.6 $ 2,531.7
Reinsurance 1,885.8 1,521.0 1,807.0
Ceded (1,666.7) (1,385.7) (1,442.7)
Net written premiums 2,942.6 2,581.9 2,896.0
Premiums earned:      
Insurance 2,565.7 2,444.8 2,370.8
Reinsurance 1,822.1 1,562.0 1,617.2
Ceded (1,498.1) (1,392.3) (1,299.3)
Net earned premiums 2,889.7 2,614.5 2,688.7
Losses and loss adjustment expenses:      
Insurance 1,759.1 1,644.5 1,574.2
Reinsurance 915.8 707.2 939.5
Ceded (957.1) (798.7) (833.7)
Losses and loss adjustment expenses $ 1,717.8 $ 1,553.0 $ 1,680.0