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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, 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 | $ 27 | $ 53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 636 | 400
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 63 | 53
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 90 | 106
Debt Securities , Available for sale, Continuous Unrealized Loss Position, | 525 | 869
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities | 1,161 | 1,269
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 12 | 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 | 2 | 8
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 | 14 | 86
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 | 5 |
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 | $ 90 | 106
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 513 | 791
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 27 | 53
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 634 | 392
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 63 | 53
Debt Securities, Available-for-sale, Unrealized Loss Position | 1,147 | 1,183
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 90 | 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 | 1 | 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 | 17 | 7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 2 | 1
Debt Securities, Available-for-sale, Unrealized Loss Position | 17 | 28
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 2 | 2
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 | 237 | 280
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 7 | 21
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 393 | 279
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 47 | 42
Debt Securities, Available-for-sale, Unrealized Loss Position | 630 | 559
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | $ 54 | 63
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions | 738 |
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 | $ 19 | 27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1 | 2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 53 | 47
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 7 | 7
Debt Securities, Available-for-sale, Unrealized Loss Position | 72 | 73
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 8 | 9
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 | 19 | 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 | 38 | 19
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 2 | 1
Debt Securities, Available-for-sale, Unrealized Loss Position | 57 | 58
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 3 | 4
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 | 120 | 132
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 14 | 19
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 12 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 2 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 132 | 132
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 16 | 19
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
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 | 3 | 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 | 10 | 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 | 115 | 36
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 3 | 1
Debt Securities, Available-for-sale, Unrealized Loss Position | 125 | 126
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 3 | 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 | 105 | 198
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 | 6 | 5
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 | 111 | 203
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | $ 5 | 5
Debt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost | 4.00% |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions | 11 |
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 | $ 15 |
Debt Securities Available for Sale Unrealized Loss Measured as a Percent of Amortized Cost | 13.00% |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions | 12 |
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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