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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Details) $ in Thousands | Sep. 30, 2025 USD ($) | Dec. 31, 2024 USD ($)
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Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | $ 7,750 | $ 51,012
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 71 | 792
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 53,832 | 68,012
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 2,616 | 4,559
Debt Securities, Available-for-sale, Unrealized Loss Position | 61,582 | 119,024
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 2,687 | 5,351
DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss, including pledged securities | 71 |
Debt Securities , Available for sale, Continuous Unrealized Loss Position, | 7,750 | 51,012
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities |  | 119,024
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 0 | 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 | 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 | 0 | 0
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 | 2,687 |
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 7,750 | 51,012
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 71 | 792
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 53,832 | 68,012
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 2,616 | 4,559
Debt Securities, Available-for-sale, Unrealized Loss Position | 61,582 | 119,024
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 2,687 | 5,351
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 | 318 | 6,042
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 15 | 112
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 6,191 | 6,582
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 264 | 458
Debt Securities, Available-for-sale, Unrealized Loss Position | 6,509 | 12,624
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 279 | 570
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 | 3,269 | 23,784
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 13 | 269
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 32,855 | 46,612
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 2,167 | 3,636
Debt Securities, Available-for-sale, Unrealized Loss Position | 36,124 | 70,396
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | $ 2,180 | 3,905
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions | 99,000 |
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 | $ 3,012 | 15,919
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 34 | 344
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 13,064 | 14,818
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 171 | 465
Debt Securities, Available-for-sale, Unrealized Loss Position | 16,076 | 30,737
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 205 | 809
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 | 2,446
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 29
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 1,722 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 14 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 1,722 | 2,446
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 14 | 29
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 | 761 | 738
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 9 | 34
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 | 761 | 738
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 9 | 34
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 | 390 | 655
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 | 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 | 390 | 655
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 2
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 | 0 | 1,428
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 | 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 | 0 | 1,428
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | $ 0 | $ 2

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