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Schedule IV - Reinsurance Premiums Written and Premiums Earned (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
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract] |  |  |  |  |  |  |
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 written premiums | 673.6 | 628.6 | 2,225.6 | 1,979.1 | 2,581.9 | 2,896.0 | 2,587.7
Gross Amount |  |  |  |  | 2,444.8 | 2,370.8 | 2,139.1
Assumed From Other Companies |  |  |  |  | 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
Percentage of Amount Assumed to Net |  |  |  |  | 59.70% | 60.10% | 61.40%
Gross written premiums | $ 1,116.8 | $ 982.7 | $ 3,598.6 | $ 3,107.9 | $ 3,967.6 | $ 4,338.7 | $ 3,938.4

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