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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	9 Months Ended
	Sep. 30, 2026	Dec. 31, 2025
Segment Reporting [Line Items]
Gross	$ 670.3
Collateral	1,076.1	$ 836.1
Net	$ 623.7
Reinsurer Concentration Risk \| Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting [Line Items]
% of Total	100.00%
Ceded Credit Risk, Secured
Segment Reporting [Line Items]
Collateral	$ 46.6
A+ or better
Segment Reporting [Line Items]
Gross	407.2
Net	$ 407.2
A+ or better \| Reinsurer Concentration Risk \| Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting [Line Items]
% of Total	65.30%
A+ or better \| Ceded Credit Risk, Secured
Segment Reporting [Line Items]
Collateral	$ 0.0
A- to A
Segment Reporting [Line Items]
Gross	207.9
Net	$ 207.9
A- to A \| Reinsurer Concentration Risk \| Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting [Line Items]
% of Total	33.30%
A- to A \| Ceded Credit Risk, Secured
Segment Reporting [Line Items]
Collateral	$ 0.0
B++ or lower and not rated
Segment Reporting [Line Items]
Gross	55.2
Net	8.6
B++ or lower and not rated \| Ark
Segment Reporting [Line Items]
Gross	(25.0)
Collateral	$ (25.0)
B++ or lower and not rated \| Reinsurer Concentration Risk \| Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting [Line Items]
% of Total	1.40%
B++ or lower and not rated \| Ceded Credit Risk, Secured
Segment Reporting [Line Items]
Collateral	$ 46.6