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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