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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Details) - USD ($) $ in Millions | Jun. 30, 2022 | Dec. 31, 2021
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Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | $ 1,041 | $ 768
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 48 | 14
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 186 | 41
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 23 | 1
Debt Securities, Available-for-sale, Unrealized Loss Position | 1,228 | 810
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 71 | 16
Debt Securities , Available for sale, Continuous Unrealized Loss Position, | 1,056 | 783
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities | 1,243 | 825
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 181 | 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 | 10 | 13
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 | 191 | 128
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
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 | 71 | 16
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 861 | 654
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 48 | 14
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 177 | 28
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 23 | 1
Debt Securities, Available-for-sale, Unrealized Loss Position | 1,037 | 682
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 71 | 16
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 | 17 | 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 | 1 | 2
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 | 18 | 118
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1 | 3
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 | 489 | 363
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 33 | 8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 117 | 17
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 19 | 1
Debt Securities, Available-for-sale, Unrealized Loss Position | 606 | 380
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 52 | 9
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 | 38 | 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 | 42 | 3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 3 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 80 | 78
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 5 | 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 | 38 | 25
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 | 4 | 2
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 | 42 | 27
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 2 | 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 | 15
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 | 15 | 15
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 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 | 100 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 5 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 1 | 1
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 | 101 | 2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 5 | 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 | 120 | 68
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 | 11 | 3
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 | 130 | 71
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 5 | 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 | 60 | 6
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 | 60 | 6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1 | 0
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

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