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Municipal Bond Guarantee Reinsurance - Narrative (Details) $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2024 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Jul. 01, 2024 USD ($)	Jun. 30, 2024
Guarantor Obligations [Line Items]
Member surplus contributions, insurance premium charges, percentage	55.00%	55.00%
Direct written premiums		$ 56.0	$ 58.6	$ 65.1
Gross earned premiums		31.7	31.2	33.3
Assets held-in-trust	$ 950.1	950.1	948.5
Distribution received		79.7
Surplus notes distribution received		58.5
Cash distribution received		21.2
Collateral trusts, investments and cash	289.4	289.4	247.0
Interest receivable	7.7	7.7	37.7
Surplus notes	300.9	300.9	322.2
Debt	562.5	562.5	564.6
Change in fair value of BAM Surplus Notes		0.5	0.0	0.0
Operating Segments \| Financial Guarantee (HG Global)
Guarantor Obligations [Line Items]
Debt	147.4	147.4	146.9
Notes receivable, fair value disclosure	381.7	381.7	0.0		$ 387.4
Change in fair value of BAM Surplus Notes		$ 0.5	0.0	$ 0.0
BAM Surplus Notes \| Surplus Notes
Guarantor Obligations [Line Items]
Basis spread on variable rate		3.00%
Interest rate					10.00%	8.20%
Cash repayments on notes		$ 30.0	27.4
Repayment of principal		21.3	17.8
Accrued interest payment held in trust		0.5	2.0
Accrued interest payment held outside of trust		8.2	7.6
Debt	300.9	300.9	322.2
Interest receivable	$ 194.8	$ 194.8	174.5
BAM Surplus Notes \| Surplus Notes \| Discount Rate
Guarantor Obligations [Line Items]
Debt instrument, measurement input	0.0810	0.0810			0.0815
Subsidiary - HG Global
Guarantor Obligations [Line Items]
Purchase of surplus notes		$ 503.0
Percentage of risk premium ceded	60.00%	60.00%
Renegotiation period		5 years
Direct written premiums	$ 31.9
Gross earned premiums	14.9
Percent over a target trust balance		102.00%
Subsidiary - HG Global \| Regulation 114
Guarantor Obligations [Line Items]
Assets held-in-trust	352.1	$ 352.1	341.6
Supplement trust target	603.0	603.0
Subsidiary - HG Global \| Supplemental Trust
Guarantor Obligations [Line Items]
Assets held-in-trust	$ 598.0	$ 598.0	$ 606.9
Subsidiary - HG Global \| Maximum
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured	15.00%	15.00%
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsured amount		$ 125.0