Municipal Bond Guarantee Insurance - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)	Jul. 01, 2024 USD ($)	Jun. 30, 2024
Guarantor Obligations [Line Items]
Member surplus contributions, insurance premium charges, percentage	55.00%		55.00%
Premiums written, gross				$ 38.1
Gross earned premiums				24.3
Assets held-in-trust	$ 1,001.6		$ 1,001.6		$ 950.1
Collateral trusts, investments and cash	306.7		306.7		289.4
Surplus notes	295.3		295.3		300.9
Interest receivable	7.3		7.3		7.7
Debt	870.4		870.4		562.5
Interest income from BAM Surplus Notes	7.5	$ 7.9	22.5	7.9
Operating Segments | Financial Guarantee (HG Global)
Guarantor Obligations [Line Items]
Debt	147.7		147.7		147.4
Interest income from BAM Surplus Notes	7.5	7.9	22.5	7.9
Notes receivable, fair value disclosure	396.2		396.2		381.7	$ 387.4
BAM Surplus Notes
Guarantor Obligations [Line Items]
Distribution received	15.4	11.6	37.6	43.8
Surplus notes distribution received	7.5		22.7
Cash distribution received	7.9		$ 14.9
BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Basis spread on variable rate			3.00%
Stated margin						10.00%	8.20%
Repayments of the BAM Surplus Notes subsequent to the deconsolidation of BAM	8.0		$ 8.0	8.0
Repayment of principal	5.6		5.6	5.1
Accrued interest payment held in trust	0.1	0.3	0.1	0.3
Accrued interest payment held outside of trust	2.3	2.6	2.3	2.6
Debt	295.3		295.3		300.9
Interest receivable	$ 214.9		$ 214.9		$ 194.8
BAM Surplus Notes | Surplus Notes | Discount Rate
Guarantor Obligations [Line Items]
Debt instrument, measurement input	0.0785		0.0785		0.0810
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
Premiums written, gross	$ 15.9	14.0	$ 41.8	14.0
Gross earned premiums	7.5	$ 7.5	$ 22.8	$ 7.5
Percent over a target trust balance			102.00%
Subsidiary - HG Global | Regulation 114
Guarantor Obligations [Line Items]
Assets held-in-trust	392.3		$ 392.3		$ 352.1
Supplement trust target	603.0		603.0
Subsidiary - HG Global | Supplemental Trust
Guarantor Obligations [Line Items]
Assets held-in-trust	$ 609.3		$ 609.3		$ 598.0
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