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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Details)
$ in Millions
Mar. 31, 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 $ 27 $ 53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 636 400
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 63 53
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 90 106
Debt Securities , Available for sale, Continuous Unrealized Loss Position, 525 869
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities 1,161 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 12 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 2 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 14 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 5  
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 $ 90 106
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 513 791
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 27 53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 634 392
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 63 53
Debt Securities, Available-for-sale, Unrealized Loss Position 1,147 1,183
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 90 106
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 1 21
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 17 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 17 28
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 2 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 237 280
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 7 21
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 393 279
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 47 42
Debt Securities, Available-for-sale, Unrealized Loss Position 630 559
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 54 63
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 738  
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 $ 19 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 53 47
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 7 7
Debt Securities, Available-for-sale, Unrealized Loss Position 72 73
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 8 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 19 40
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1 3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 38 19
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 57 58
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 3 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 120 132
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 14 19
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 12 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 2 0
Debt Securities, Available-for-sale, Unrealized Loss Position 132 132
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 16 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 3 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 3 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 10 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 115 36
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 125 126
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 3 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 105 198
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 4 4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 6 5
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 111 203
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 5 5
Debt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost 4.00%  
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 11  
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 $ 15  
Debt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost 13.00%  
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 12  
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