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Municipal Bond Guarantee Insurance - Narrative (Details) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2018	Jan. 01, 2025	Sep. 30, 2022	Jan. 31, 2021
Guarantor Obligations [Line Items]
Percentage of risk premium ceded	60.00%			60.00%
Assets held-in-trust		$ 856.9			$ 856.9
Collateral trusts, investments and cash	$ 588.6	503.3		$ 588.6	503.3
Surplus notes	322.2	340.0		322.2	340.0
Interest receivable	37.7	13.6		37.7	13.6
Carrying Value	564.6	575.2		564.6	575.2
Subsidiary - HG Global
Guarantor Obligations [Line Items]
Purchase of surplus notes				$ 503.0
Renegotiation period				5 years
Percent over a target trust balance				102.00%
Subsidiary - HG Global \| Regulation 114
Guarantor Obligations [Line Items]
Assets held-in-trust	341.6	288.6		$ 341.6	288.6
Supplement trust target	603.0			603.0
Subsidiary - HG Global \| Supplemental Trust
Guarantor Obligations [Line Items]
Assets held-in-trust	606.9	568.3		606.9	568.3
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company
Guarantor Obligations [Line Items]
Guaranteed contracts, par value									$ 42.5	$ 805.5
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Issuance of insurance linked securities					$ 150.0	$ 150.0	$ 100.0
Insurance-linked securities, term					12 years	12 years	12 years
Insurance-linked securities, callable term					7 years	5 years	5 years
Percentage of par value of policy reinsured					90.00%	90.00%	90.00%
Reinsurance, limit on total reimbursement					$ 150.0	$ 150.0	$ 100.0
Reinsured amount				$ 125.0
2018 Covered Portfolio \| Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				23.00%
2021 Covered Portfolio \| Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				27.00%
2022 Covered Portfolio \| Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered				29.00%
BAM Surplus Notes \| Surplus Notes
Guarantor Obligations [Line Items]
Cash repayments on notes	27.4	36.0
Accrued interest payment held in trust	2.0	1.0
Repayment of principal	17.8	24.6
Accrued interest payment held outside of trust	7.6	10.4
Carrying Value	322.2	340.0		$ 322.2	340.0
Interest receivable	$ 174.5	$ 157.9		$ 174.5	157.9
Forecast \| US Treasury (UST) Interest Rate \| BAM Surplus Notes \| Surplus Notes
Guarantor Obligations [Line Items]
Basis spread on variable rate			3.00%
Interest rate			8.20%					8.00%
Maximum \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance, maximum amount reinsured					276.7	301.7	276.1
Maximum \| Subsidiary - HG Global
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured	15.00%			15.00%
Minimum \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance obligation					$ 110.0	$ 135.0	$ 165.0