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Third-Party Reinsurance - Remaining Gross and Net Reinsurance Recoverables (Details) - P&C Insurance and Reinsurance (Ark/WM Outrigger) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2024 | Dec. 31, 2023
Segment Reporting Information [Line Items] |  |
Reinsurance recoverables | $ 425.2 |
Collateral | 695.0 | $ 442.0
Net | 347.9 |
Reinsurance recoverables on unpaid losses (1) | 404.5 | 340.8
Ark |  |
Segment Reporting Information [Line Items] |  |
Reinsurance recoverables on unpaid losses (1) | $ (15.8) | $ (15.6)
Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid |  |
Segment Reporting Information [Line Items] |  |
% of Total | 100.00% |
Ceded Credit Risk, Secured |  |
Segment Reporting Information [Line Items] |  |
Collateral | $ 77.3 |
A+ or better |  |
Segment Reporting Information [Line Items] |  |
Reinsurance recoverables | 202.2 |
Net | $ 202.2 |
A+ or better | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid |  |
Segment Reporting Information [Line Items] |  |
% of Total | 58.10% |
A+ or better | Ceded Credit Risk, Secured |  |
Segment Reporting Information [Line Items] |  |
Collateral | $ 0.0 |
A- to A |  |
Segment Reporting Information [Line Items] |  |
Reinsurance recoverables | 139.1 |
Net | $ 139.1 |
A- to A | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid |  |
Segment Reporting Information [Line Items] |  |
% of Total | 40.00% |
A- to A | Ceded Credit Risk, Secured |  |
Segment Reporting Information [Line Items] |  |
Collateral | $ 0.0 |
B++ or lower and not rated |  |
Segment Reporting Information [Line Items] |  |
Reinsurance recoverables | 83.9 |
Net | $ 6.6 |
B++ or lower and not rated | Reinsurer Concentration Risk | Reinsurance Recoverable Including Reinsurance Premium Paid |  |
Segment Reporting Information [Line Items] |  |
% of Total | 1.90% |
B++ or lower and not rated | Ceded Credit Risk, Secured |  |
Segment Reporting Information [Line Items] |  |
Collateral | $ 77.3 |

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