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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 |  |

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