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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, 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 $ 305 $ 187
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 839 844
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 70 68
Debt Securities, Available-for-sale, Unrealized Loss Position 1,145 1,032
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 75 71
Debt Securities , Available for sale, Continuous Unrealized Loss Position, 332 187
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities 5 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 44 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 44 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 4,000  
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 $ 74 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 261 184
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 839 844
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 70 68
Debt Securities, Available-for-sale, Unrealized Loss Position 1,101 1,028
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 74 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 13 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 16 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 29 23
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 1 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 91 75
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 2 2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 504 509
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 44 43
Debt Securities, Available-for-sale, Unrealized Loss Position 596 584
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax $ 45 44
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 671,000  
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 $ 9 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 57 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 66 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 40 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 33 37
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 2 2
Debt Securities, Available-for-sale, Unrealized Loss Position 73 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 27 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 27 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 42 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 111 98
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 12 14
Debt Securities, Available-for-sale, Unrealized Loss Position 153 104
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 12 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 0 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 0 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 13 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 40 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 53 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 53 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 79 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 132 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 3.00%  
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 14,000  
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 $ 11  
Debt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost 11.00%  
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions 13,000  
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