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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
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Premiums written:              
Direct         $ 2,446.6 $ 2,531.7 $ 2,341.4
Assumed         1,521.0 1,807.0 1,597.0
Ceded $ (443.2) $ (354.1) $ (1,373.0) $ (1,128.8) (1,385.7) (1,442.7) (1,350.7)
Net premiums written 673.6 628.6 2,225.6 1,979.1 2,581.9 2,896.0 2,587.7
Premiums earned:              
Direct         2,444.8 2,370.8 2,139.1
Assumed         1,562.0 1,617.2 1,479.2
Ceded (403.9) (361.2) (1,105.0) (1,033.1) (1,392.3) (1,299.3) (1,207.8)
Net premiums earned 698.3 660.0 2,069.4 1,947.4 2,614.5 2,688.7 2,410.5
Insurance losses and loss adjustment expenses:              
Direct 490.9 542.8 1,259.1 1,234.5 1,644.5 1,574.2 1,499.8
Assumed 291.6 181.1 692.7 534.5 707.2 939.5 1,000.6
Ceded (308.3) (293.4) (672.9) (621.4) (798.7) (833.7) (807.1)
Net insurance losses and loss adjustment expenses $ 474.2 $ 430.5 $ 1,278.9 $ 1,147.6 $ 1,553.0 $ 1,680.0 $ 1,693.3