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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	6 Months Ended
	Jun. 30, 2026	Dec. 31, 2025
Segment Reporting [Line Items]
Gross	$ 727.0
Collateral	1,394.0	$ 836.1
Net	$ 681.7
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	$ 45.3
A+ or better
Segment Reporting [Line Items]
Gross	467.8
Net	$ 467.8
A+ or better \| Reinsurer Concentration Risk \| Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting [Line Items]
% of Total	68.60%
A+ or better \| Ceded Credit Risk, Secured
Segment Reporting [Line Items]
Collateral	$ 0.0
A- to A
Segment Reporting [Line Items]
Gross	202.6
Net	$ 202.6
A- to A \| Reinsurer Concentration Risk \| Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting [Line Items]
% of Total	29.70%
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	56.6
Net	11.3
B++ or lower and not rated \| Ark
Segment Reporting [Line Items]
Gross	(27.6)
Collateral	$ (27.6)
B++ or lower and not rated \| Reinsurer Concentration Risk \| Reinsurance Recoverable Including Reinsurance Premium Paid
Segment Reporting [Line Items]
% of Total	1.70%
B++ or lower and not rated \| Ceded Credit Risk, Secured
Segment Reporting [Line Items]
Collateral	$ 45.3