Municipal Bond Guarantee Insurance - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	Dec. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2018	Jan. 01, 2025	Dec. 31, 2024
Guarantor Obligations [Line Items]
Assets held-in-trust	$ 948,500,000	$ 933,100,000
Collateral trusts, investments and cash	588,600,000	592,800,000
Surplus notes	322,200,000	322,200,000
Interest receivable	37,700,000	18,100,000
Debt	534,600,000	532,100,000
Subsidiary - HG Global
Guarantor Obligations [Line Items]
Purchase of surplus notes		$ 503,000,000
Percentage of risk premium ceded		60.00%
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	341,600,000	$ 330,700,000
Supplement trust target		603,000,000
Subsidiary - HG Global | Supplemental Trust
Guarantor Obligations [Line Items]
Assets held-in-trust	606,900,000	602,400,000
Variable Interest Entity, Primary Beneficiary - Build America Mutual Assurance Company | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Issuance of insurance linked securities				$ 150,000,000	$ 150,000,000	$ 100,000,000
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				$ 150,000,000	$ 150,000,000	$ 100,000,000
Reinsured amount		$ 125,000,000
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		21.00%
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		28.00%
BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Repayments of unsecured debt	27,400,000	$ 0	$ 0
Accrued interest payment held in trust	2,000,000
Repayment of principal	17,800,000
Accrued interest payment held outside of trust	7,600,000
Debt	322,200,000	322,200,000
Interest receivable	$ 174,500,000	$ 181,100,000
US Treasury (UST) Interest Rate | BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Basis spread on variable rate		3.00%
Forecast | US Treasury (UST) Interest Rate | BAM Surplus Notes | Surplus Notes
Guarantor Obligations [Line Items]
Interest rate							8.00%	8.20%
Maximum | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance, maximum amount reinsured				276,700,000	301,700,000	276,100,000
Maximum | Subsidiary - HG Global
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured		15.00%
Minimum | Fidus Re Ltd.
Guarantor Obligations [Line Items]
Reinsurance obligation				$ 110,000,000	$ 135,000,000	$ 165,000,000