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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, 2022 USD ($) | Dec. 31, 2021 USD ($)
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Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | $ 768
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ 53 | 14
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 400 | 41
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 53 | 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 810
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 106 | 16
Debt Securities Accumulated Gross Unrealized Loss Before Tax Included Pledged Securities |  | 16
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities | 1,269 | 825
Debt Securities , Available for sale, Continuous Unrealized Loss Position, | 869 |
Available-for-sale Securities | $ 1,966 |
Average Percentage Loss Versus Amortized Cost | 10.00% |
Average Percentage of Unrealized Loss Versus Amortized Cost for Below Investment Grade Securities | 8.00% |
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | $ 78 | 114
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 | 8 | 13
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 | 86 | 128
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Available-for-sale Securities | $ 507 | 414
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] |  |
Available-for-sale Securities | $ 1,902 | 2,145
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 791 | 654
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 53 | 14
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 392 | 28
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 53 | 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 1,183 | 682
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 106 | 16
Available-for-sale Securities | 1,395 | 1,730
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 | 117
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 | 7 | 2
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 28 | 118
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 3
Available-for-sale Securities | 43 | 340
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 | 25
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 3 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 19 | 2
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 58 | 27
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4 | 1
Available-for-sale Securities | 65 | 45
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 |  | 15
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments |  | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 15
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | 0
Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than 12 Months Including Pledged Securities |  | 783
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 | 280 | 363
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 21 | 8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 279 | 17
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 42 | 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 559 | 380
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 63 | 9
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions | 584 |
Available-for-sale Securities | $ 598 | 613
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 | 61 |
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 | 2 |
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 | 27 | 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 | 47 | 3
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 7 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 73 | 78
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 9 | 2
Available-for-sale Securities | 76 | 87
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 | 132 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 19 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 1
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 | 132 | 2
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 19 | 0
Available-for-sale Securities | 238 | 252
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 | 90 | 68
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 3 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 36 | 3
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 126 | 71
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4 | 0
Available-for-sale Securities | 137 | 128
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 | 198 | 6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 4 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 5 | 0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 203 | 6
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 5 | 0
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions | 14 |
Available-for-sale Securities | $ 224 | 265
Dbt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost | 2.00% |
Fixed Income Securities [Member] | Ambac Insured Securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 18 |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions | 13 |
Dbt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost | 15.00% |
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 |
Available-for-sale Securities | 15 |
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 |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 |
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 3 |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 |
Collateral Pledged [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities | 64 | 120
Collateral Pledged [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities | 0 | 15
Collateral Pledged [Member] | Fixed Income Securities [Member] | Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities | $ 64 | $ 105

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