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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 |  |  |

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