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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 | Mar. 31, 2026 | Dec. 31, 2025 | Mar. 31, 2025 | Dec. 31, 2024
--- | --- | --- | --- | ---
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | $ 487,261 | $ 459,990 |  |
Liability for Claims | 487,261 | 459,990 |  |
Ceded Loss And Loss Expenses Paid Not Yet Recovered |  |  | $ 56,190 |
Short-Duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net | 95,908 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 391,352 | $ 375,722 | $ 294,920 | $ 270,081
Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 391,352 |  |  |
Operating Segments | Specialty Property & Casualty Program |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 487,261 |  |  |
Liability for Claims | 487,261 |  |  |
Operating Segments | Commercial Auto |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 162,853 |  |  |
Liability for Claims | 162,853 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 133,407 |  |  |
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 | 29,446 |  |  |
Operating Segments | Excess and General Liability |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 128,381 |  |  |
Liability for Claims | 128,381 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 109,617 |  |  |
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 | 18,764 |  |  |
Operating Segments | Workers' Compensation Insurance |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 19,752 |  |  |
Liability for Claims | 19,752 |  |  |
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 | 19,752 |  |  |
Operating Segments | Non-standard Personal Auto |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 2,895 |  |  |
Liability for Claims | 2,895 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 191 |  |  |
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 | 2,704 |  |  |
Operating Segments | General Liability |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 62,391 |  |  |
Liability for Claims | 62,391 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 49,641 |  |  |
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 | 12,750 |  |  |
Operating Segments | Professional Liability Insurance |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 48,219 |  |  |
Liability for Claims | 48,219 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 44,831 |  |  |
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 | 3,388 |  |  |
Operating Segments | Surety Product Line |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 10,767 |  |  |
Liability for Claims | 10,767 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 9,707 |  |  |
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 | 1,060 |  |  |
Operating Segments | Multi-peril |  |  |  |
Financial Guarantee Insurance Contracts, Claim Liability [Line Items] |  |  |  |
Liability for Claims and Claims Adjustment Expense | 11,246 |  |  |
Liability for Claims | 11,246 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 8,918 |  |  |
Operating Segments | Multi-peril | 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 | 2,328 |  |  |
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 | 15,515 |  |  |
Liability for Claims | 15,515 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 9,969 |  |  |
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 | 5,546 |  |  |
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 | 25,242 |  |  |
Liability for Claims | 25,242 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 25,071 |  |  |
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 | $ 171 |  |  |

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