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Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Phantom) (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | --- | --- | ---
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | $ 437,539 | $ 349,062 |  |
Liability for Claims | 437,539 | 349,062 |  |
Ceded Loss And Loss Expenses Paid Not Yet Recovered |  |  | $ 41,225 |
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net | 82,764 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 354,775 | $ 270,081 | $ 242,800 | $ 156,301
Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 354,775 |  |  |
Operating Segments | Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 437,539 |  |  |
Liability for Claims | 437,539 |  |  |
Operating Segments | Commercial Auto |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 159,820 |  |  |
Liability for Claims | 159,820 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 134,859 |  |  |
Operating Segments | Commercial Auto | Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net | 24,961 |  |  |
Operating Segments | Excess and General Liability |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 97,111 |  |  |
Liability for Claims | 97,111 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 83,086 |  |  |
Operating Segments | Excess and General Liability | Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net | 14,025 |  |  |
Operating Segments | Workers' Compensation Insurance |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 18,086 |  |  |
Liability for Claims | 18,086 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 0 |  |  |
Operating Segments | Workers' Compensation Insurance | Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net | 18,086 |  |  |
Operating Segments | Non-standard Personal Auto |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 5,390 |  |  |
Liability for Claims | 5,390 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 195 |  |  |
Operating Segments | Non-standard Personal Auto | Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net | 5,195 |  |  |
Operating Segments | General Liability |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 55,136 |  |  |
Liability for Claims | 55,136 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 44,175 |  |  |
Operating Segments | General Liability | Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net | 10,961 |  |  |
Operating Segments | Professional Liability Insurance |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 38,365 |  |  |
Liability for Claims | 38,365 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 35,473 |  |  |
Operating Segments | Professional Liability Insurance | Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net | 2,892 |  |  |
Operating Segments | Surety Product Line |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 15,210 |  |  |
Liability for Claims | 15,210 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 15,216 |  |  |
Operating Segments | Surety Product Line | Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net | (6) |  |  |
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Unallocated Loss Adjustment Expense Reserve |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 15,638 |  |  |
Liability for Claims | 15,638 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 9,966 |  |  |
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Unallocated Loss Adjustment Expense Reserve | Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net | 5,672 |  |  |
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Other Insurance Contracts |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 32,783 |  |  |
Liability for Claims | 32,783 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 31,805 |  |  |
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Other Insurance Contracts | Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net | $ 978 |  |  |

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