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

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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