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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, 2023
Dec. 31, 2022
Dec. 31, 2021
Premiums written:      
Insurance $ 2,446.6 $ 2,531.7 $ 2,341.4
Reinsurance 1,521.0 1,807.0 1,597.0
Ceded (1,385.7) (1,442.7) (1,350.7)
Net written premiums 2,581.9 2,896.0 2,587.7
Premiums earned:      
Insurance 2,444.8 2,370.8 2,139.1
Reinsurance 1,562.0 1,617.2 1,479.2
Ceded (1,392.3) (1,299.3) (1,207.8)
Net earned premiums 2,614.5 2,688.7 2,410.5
Losses and loss adjustment expenses:      
Insurance 1,644.5 1,574.2 1,499.8
Reinsurance 707.2 939.5 1,000.6
Ceded (798.7) (833.7) (807.1)
Losses and loss adjustment expenses $ 1,553.0 $ 1,680.0 $ 1,693.3