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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Details) - USD ($) $ in Thousands | Mar. 31, 2026 | Dec. 31, 2025
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Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | $ 40,146 | $ 5,311
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 441 | 48
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 39,782 | 47,658
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 2,305 | 2,268
Debt Securities, Available-for-sale, Unrealized Loss Position | 79,928 | 52,969
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 2,746 | 2,316
DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss, including pledged securities | 441 |
Debt Securities , Available for sale, Continuous Unrealized Loss Position, | 40,146 | 5,311
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities |  | 52,969
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 | 2,746 |
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 40,146 | 5,311
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 441 | 48
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 39,782 | 47,658
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 2,305 | 2,268
Debt Securities, Available-for-sale, Unrealized Loss Position | 79,928 | 52,969
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 2,746 | 2,316
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 | 532 | 535
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 20 | 16
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 4,904 | 5,820
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 197 | 212
Debt Securities, Available-for-sale, Unrealized Loss Position | 5,436 | 6,355
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 217 | 228
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 | 22,033 | 1,807
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 249 | 10
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 26,654 | 30,751
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 2,000 | 1,932
Debt Securities, Available-for-sale, Unrealized Loss Position | 48,687 | 32,558
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 2,249 | 1,942
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 | 14,522 | 2,206
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 124 | 14
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 6,770 | 9,491
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 94 | 117
Debt Securities, Available-for-sale, Unrealized Loss Position | 21,292 | 11,697
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 218 | 131
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 | 473 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 8 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 1,454 | 1,596
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 13 | 7
Debt Securities, Available-for-sale, Unrealized Loss Position | 1,927 | 1,596
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 21 | 7
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 | 2,518 | 763
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 40 | 8
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 | 2,518 | 763
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 40 | 8
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 | 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 | 1 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 68 | 0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1 | 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 | 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

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