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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) $ in Millions | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($)
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Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ 4 | $ 53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 844 | 400
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 68 | 53
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 71 | 106
Debt Securities Accumulated Gross Unrealized Loss Before Tax Included Pledged Securities |  | 106
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities | 1,032 | 1,269
Debt Securities , Available for sale, Continuous Unrealized Loss Position, | 187 |
Available-for-sale Securities | $ 1,710 | 1,395
Average Percentage Loss Versus Amortized Cost | 7.00% |
Average Percentage of Unrealized Loss Versus Amortized Cost for Below Investment Grade Securities | 5.00% |
Municipal Bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 1 | 2
Available-for-sale Securities | 72 | 43
US Government Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4 | 4
Available-for-sale Securities | 82 | 65
Corporate Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 44 | 63
Available-for-sale Securities | 745 | 598
Debt Security, Government, Non-US [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 6 | 9
Available-for-sale Securities | 100 | 76
Residential Mortgage-Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 14 | 19
Available-for-sale Securities | 250 | 238
Collateralized Debt Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 | 4
Available-for-sale Securities | 139 | 137
Asset-backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 | 5
Available-for-sale Securities | 303 | 224
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 4 | 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
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 4 | 86
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Available-for-sale Securities | $ 426 | 507
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 |
Commercial Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 0 | 0
Available-for-sale Securities | 19 | 0
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 184 | 791
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 4 | 53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 844 | 392
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 68 | 53
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 1,028 | 1,183
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 71 | 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 | 7 | 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 | 16 | 7
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1 | 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 23 | 28
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 | 2
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 | 27 | 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 | 37 | 19
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 2 | 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 63 | 58
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 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 Including Pledged Securities |  | 869
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 | 75 | 280
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 2 | 21
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 509 | 279
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 43 | 42
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 584 | 559
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 44 | 63
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions | 662,000 |
Fixed Income Securities [Member] | Corporate Debt Securities [Member] | External Credit Rating, Investment Grade |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 44 |
Fixed Income Securities [Member] | Corporate Debt Securities [Member] | External Credit Rating, Non Investment Grade [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 |
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 | 8 | 27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 56 | 47
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 6 | 7
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 64 | 73
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 6 | 9
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 | 6 | 132
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 19
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 98 | 0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 14 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 104 | 132
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 14 | 19
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 | 1 | 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 | 93 | 36
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1 | 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 95 | 126
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 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 | 57 | 198
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1 | 4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 35 | 5
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1 | 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 92 | 203
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 | 5
Fixed Income Securities [Member] | Ambac Insured Securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 13 |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions | 11,000 |
Dbt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost | 14.00% |
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
Fixed Income Securities [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
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
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 3 | 3
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Collateral Pledged [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Available-for-sale Securities | 27 | 64
Collateral Pledged [Member] | Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Available-for-sale Securities | $ 27 | $ 64

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