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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Details)
$ in Millions
Jun. 30, 2024
USD ($)
Dec. 31, 2023
USD ($)
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months $ 372 $ 187
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 5 4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 787 844
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 71 68
Debt Securities, Available-for-sale, Unrealized Loss Position 1,159 1,032
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 78 71
DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss, including pledged securities 7  
Debt Securities , Available for sale, Continuous Unrealized Loss Position, 398 187
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities   1,032
Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 46 4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Unrealized Loss Position 46 4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 0 0
Ambac Insured Securities | Parsed Fixed Income Securities    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 5,000  
Fixed Income Investments And Other Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 77 71
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 326 184
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 5 4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 787 844
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 71 68
Debt Securities, Available-for-sale, Unrealized Loss Position 1,113 1,028
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 77 71
Fixed Income Securities [Member] | Municipal Bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 21 7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 15 16
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 1 1
Debt Securities, Available-for-sale, Unrealized Loss Position 35 23
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 2 1
Fixed Income Securities [Member] | Corporate Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 125 75
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 3 2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 493 509
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 43 43
Debt Securities, Available-for-sale, Unrealized Loss Position 618 584
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 46 44
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 733,000  
Fixed Income Securities [Member] | Debt Security, Government, Non-US [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months $ 25 8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 55 56
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 7 6
Debt Securities, Available-for-sale, Unrealized Loss Position 80 64
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 7 6
Fixed Income Securities [Member] | US Government Debt Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 58 27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1 1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 37 37
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 3 2
Debt Securities, Available-for-sale, Unrealized Loss Position 95 63
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 4 4
Fixed Income Securities [Member] | US Government Debt Securities [Member] | Asset Pledged as Collateral [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 25 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Unrealized Loss Position 25 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 1 0
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 16 6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 113 98
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 13 14
Debt Securities, Available-for-sale, Unrealized Loss Position 129 104
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 13 14
Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 17 3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Unrealized Loss Position 17 3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 0 0
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 18 1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 3 93
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 1
Debt Securities, Available-for-sale, Unrealized Loss Position 22 95
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 0 1
Fixed Income Securities [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 45 57
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1 1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 72 35
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 4 1
Debt Securities, Available-for-sale, Unrealized Loss Position 117 92
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 5 1
Debt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost 5.00%  
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 13,000  
Fixed Income Securities [Member] | Ambac Insured Securities    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 13  
Debt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost 12.00%  
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 15,000  
Collateral Pledged [Member] | Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 1 0
Collateral Pledged [Member] | Fixed Income Securities [Member] | Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 0 $ 0