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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Details)
$ in Millions
Sep. 30, 2023
USD ($)
Dec. 31, 2022
USD ($)
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss $ 20 $ 53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 849 400
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 102 53
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 122 106
Debt Securities , Available for sale, Continuous Unrealized Loss Position, 476 869
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities 1,325 1,269
Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 19 78
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 8
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 19 86
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 6,000  
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 $ 122 106
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 456 791
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 20 53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 849 392
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 102 53
Debt Securities, Available-for-sale, Unrealized Loss Position 1,306 1,183
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 122 106
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 50 21
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 16 7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 2 1
Debt Securities, Available-for-sale, Unrealized Loss Position 65 28
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 3 2
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 150 280
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 5 21
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 497 279
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 60 42
Debt Securities, Available-for-sale, Unrealized Loss Position 648 559
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 65 63
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 803,000  
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 27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1 2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 52 47
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 9 7
Debt Securities, Available-for-sale, Unrealized Loss Position 77 73
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 9 9
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 28 40
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 2 3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 37 19
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 3 1
Debt Securities, Available-for-sale, Unrealized Loss Position 64 58
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 4 4
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 58 132
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 2 19
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 107 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 25 0
Debt Securities, Available-for-sale, Unrealized Loss Position 165 132
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 28 19
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 18 3
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 18 3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 1 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 7 90
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 107 36
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 114 126
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 1 4
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 120 198
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 9 4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 34 5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 2 1
Debt Securities, Available-for-sale, Unrealized Loss Position 154 203
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 11 5
Debt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost 7.00%  
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 16,000  
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 $ 26  
Debt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost 17.00%  
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 16,000  
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 $ 0 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