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Reserves for Losses and Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses (LAE) Reserves (Details) - USD ($) $ in Millions |  | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jan. 10, 2022 | Sep. 30, 2024 | Sep. 30, 2023 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | May 20, 2022 | Dec. 31, 2020 | Mar. 02, 2020
Insurance [Abstract] |  |  |  |  |  |  |  |  |
Provision for losses and LAE, beginning balance |  | $ 7,810.6 | $ 7,710.9 | $ 7,710.9 | $ 7,611.8 | $ 7,165.3 |  |  |
Less reinsurance recoverable |  |  |  | (4,577.8) | (4,897.7) | (3,298.1) |  | $ (3,195.2) |
Net losses and LAE reserves, beginning balance |  | 3,232.8 | 2,813.2 | 2,813.2 | 4,313.7 | 3,970.1 |  |  |
Movement in net provision for losses and LAE for claims incurred: |  |  |  |  |  |  |  |  |
Current year |  | 1,239.2 | 1,137.1 | 1,492.2 | 1,651.9 | 1,648.2 |  |  |
Prior years |  | (39.7) | (10.5) | (60.8) | (28.1) | (45.1) |  |  |
Total incurred |  |  |  | 1,553.0 | 1,680.0 | 1,693.3 |  |  |
Net Losses and LAE payments for claims incurred: |  |  |  |  |  |  |  |  |
Current year |  | (109.9) | (87.2) | (161.1) | (192.7) | (729.1) |  |  |
Prior years |  | (655.0) | (694.7) | (1,011.9) | (1,098.4) | (580.7) |  |  |
Total paid |  | (764.9) | (781.9) | (1,173.0) | (1,291.1) | (1,309.8) |  |  |
Foreign exchange (gains)/losses |  | 23.2 | (4.8) | 39.6 | (49.3) | (39.9) |  |  |
Net losses and LAE reserves, ending balance |  | 3,770.0 | 3,174.1 | 3,232.8 | 2,813.2 | 4,313.7 |  |  |
Plus reinsurance recoverable on unpaid losses at the end of the year |  |  |  | 4,577.8 | 4,897.7 | 3,298.1 |  |  |
Provision for losses and LAE at the end of the year |  | 8,201.3 | 7,770.8 | 7,810.6 | 7,710.9 | 7,611.8 |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |  |  |  |
Net loss and LAE expenses disposed (1) |  |  |  | 0.0 | 1,840.1 | 0.0 |  |  |
Plus reinsurance recoverable on unpaid losses at the end of the year |  |  |  | 4,577.8 | 4,897.7 | 3,298.1 |  |  |
Reinsurance Contracts [Axis]: LPT |  |  |  |  |  |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |  |  |  |
Reinsurance recoverable total |  |  |  |  |  |  | $ 2,610.1 |  |
Insurance |  |  |  |  |  |  |  |  |
Insurance [Abstract] |  |  |  |  |  |  |  |  |
Provision for losses and LAE, beginning balance |  | 1,859.7 | 1,452.5 | 1,452.5 | 2,165.3 |  |  |  |
Net losses and LAE reserves, beginning balance |  | 1,859.7 | 1,452.5 | 1,452.5 | 2,165.3 |  |  |  |
Movement in net provision for losses and LAE for claims incurred: |  |  |  |  |  |  |  |  |
Prior years |  | (18.2) | (38.8) |  |  |  |  |  |
Net Losses and LAE payments for claims incurred: |  |  |  |  |  |  |  |  |
Net losses and LAE reserves, ending balance |  |  |  | 1,859.7 | 1,452.5 | 2,165.3 |  |  |
Provision for losses and LAE at the end of the year |  | 2,115.0 | 1,769.3 | $ 1,859.7 | $ 1,452.5 | $ 2,165.3 |  |  |
Insurance | Reinsurance Contracts [Axis]: LPT, deferred gain |  |  |  |  |  |  |  |  |
Movement in net provision for losses and LAE for claims incurred: |  |  |  |  |  |  |  |  |
Prior years |  | (17.6) | (23.7) |  |  |  |  |  |
Insurance | Reinsurance Contracts [Axis]: LPT, offset post-LPT years |  |  |  |  |  |  |  |  |
Movement in net provision for losses and LAE for claims incurred: |  |  |  |  |  |  |  |  |
Prior years |  | $ (0.6) | $ (15.1) |  |  |  |  |  |
Enstar |  |  |  |  |  |  |  |  |
Net Losses and LAE payments for claims incurred: |  |  |  |  |  |  |  |  |
Plus reinsurance recoverable on unpaid losses at the end of the year |  |  |  |  |  |  |  |  | $ 770.0
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |  |  |  |
Plus reinsurance recoverable on unpaid losses at the end of the year |  |  |  |  |  |  |  |  | $ 770.0
Enstar | Reinsurance Contracts [Axis]: LPT net losses reinsured |  |  |  |  |  |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |  |  |  |
Reinsurance, Excess Retention, Amount Reinsured, Per Event | $ 3,120.0 |  |  |  |  |  |  |  |

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