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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020
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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 | $ 816
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 14 | 7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 41 | 25
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 | 810 | 841
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 16 | 8
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities | 825 |
Debt Securities , Available for sale, Continuous Unrealized Loss Position, | 783 |
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 114 | 187
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 | 13 | 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 | 128 | 187
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 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 | 654 | 629
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 14 | 7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 28 | 25
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 | 682 | 654
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 16 | 8
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 | 117 | 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 | 2 | 6
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 | 118 | 31
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 3 | 0
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 | 25 | 17
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 | 2 | 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 | 27 | 17
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 | 1
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 |
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 | 363 | 543
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 8 | 6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 17 | 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 | 380 | 543
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 9 | 6
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 | 75 | 3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 2 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 3 | 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 | 78 | 3
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 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 | 0 | 14
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 | 1 | 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 | 2 | 14
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 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 | 68 | 27
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 | 3 | 15
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 | 71 | 42
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
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 | 6 | 0
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 | 4
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 | 6 | 4
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 0 | $ 0

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