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Variable Interest Entities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2025	Dec. 31, 2024	Jul. 01, 2024	Jun. 30, 2024
Variable Interest Entity [Line Items]
Assets held-in-trust	$ 1,001.6	$ 950.1
Bamboo CRV
Variable Interest Entity [Line Items]
Expected capital contributions	6.0
BAM Surplus Notes \| Surplus Notes
Variable Interest Entity [Line Items]
Stated margin			10.00%	8.20%
Subsidiary - HG Global
Variable Interest Entity [Line Items]
Purchase of surplus notes	$ 503.0
Subsidiary - HG Global \| Maximum
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured	15.00%
Outrigger Re
Variable Interest Entity [Line Items]
Short-term investments, at fair value	$ 228.4	$ 203.7
PassportCard/DavidShield \| Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Ownership interest	53.80%
Fully diluted ownership interest	51.50%
Partnership interest, carrying value	$ 169.4
BroadStreet \| Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Ownership interest	10.90%
Partnership interest, carrying value	$ 150.0
Elementum \| Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Ownership interest	26.60%
Fully diluted ownership interest	25.40%
Partnership interest, carrying value	$ 35.0
Limited Partnerships \| Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Partnership interest, carrying value	$ 246.1