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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Third-Party Reinsurance - Remaining Gross and Net Reinsurance Recoverables (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Gross	$ 491.9
Net	417.4
Reinsurance recoverable on unpaid losses	$ 434.4	$ 340.8	$ 505.0	$ 428.9
P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	100.00%
P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 74.5
Ark Insurance Holdings Limited/Outrigger
Segment Reporting Information [Line Items]
Collateral	589.0	442.0
Reinsurance recoverable on unpaid losses	434.4	340.8
Ark Insurance Holdings Limited/Outrigger \| Ark Insurance Holdings Limited - Segment
Segment Reporting Information [Line Items]
Reinsurance recoverable on unpaid losses	(31.8)	$ (15.6)
A+ or better \| P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Gross	310.3
Net	$ 310.3
A+ or better \| P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	74.30%
A+ or better \| P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0.0
A- to A \| P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Gross	98.5
Net	$ 98.5
A- to A \| P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	23.60%
A- to A \| P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 0.0
B++ or lower and not rated \| P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Gross	83.1
Net	$ 8.6
B++ or lower and not rated \| P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Reinsurance Recoverable Including Reinsurance Premium Paid \| Reinsurer Concentration Risk
Segment Reporting Information [Line Items]
% of Total	2.10%
B++ or lower and not rated \| P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Ceded Credit Risk, Secured
Segment Reporting Information [Line Items]
Collateral	$ 74.5
B++ or lower and not rated \| Ark Insurance Holdings Limited/Outrigger \| Ark Insurance Holdings Limited - Segment
Segment Reporting Information [Line Items]
Gross	(34.9)
Collateral	$ (34.9)