Third-Party Reinsurance - Remaining Gross and Net Reinsurance Recoverables (Details) - P&C Insurance and Reinsurance (Ark/WM Outrigger) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Reinsurance recoverables	$ 535.1
Collateral	748.0	$ 442.0
Net	454.0
Reinsurance recoverables on unpaid losses	507.2	340.8
Ark
Segment Reporting Information [Line Items]
Reinsurance recoverables on unpaid losses	$ (28.0)	$ (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	$ 81.1
A+ or better
Segment Reporting Information [Line Items]
Reinsurance recoverables	358.1
Net	$ 358.1
A+ or better \| Reinsurer Concentration Risk \| Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	78.90%
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	84.2
Net	$ 84.2
A- to A \| Reinsurer Concentration Risk \| Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	18.50%
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	92.8
Net	$ 11.7
B++ or lower and not rated \| Reinsurer Concentration Risk \| Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting Information [Line Items]
% of Total	2.60%
B++ or lower and not rated \| Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 81.1

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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