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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
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Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | $ 144,371 | $ 1,139,101
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 942 | 27,298
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 70,611 | 359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1,017 | 10,897
Debt Securities, Available-for-sale, Unrealized Loss Position | 214,982 | 1,499,044
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1,959 | 38,195
Below Investment Grade Securities and Non-Rated Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Fair Value | 45,718 |
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses | 573 |
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 4,695 | 115,374
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 2 | 97
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 | 4,695 | 115,374
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 2 | 97
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 | 1,959 |
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 139,676 | 1,023,727
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 940 | 27,201
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 70,611 | 359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1,017 | 10,897
Debt Securities, Available-for-sale, Unrealized Loss Position | 210,287 | 1,383,670
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1,957 | 38,098
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 | 21,593 | 537,904
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 325 | 15,878
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 25,981 | 28,533
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 584 | 1,198
Debt Securities, Available-for-sale, Unrealized Loss Position | 47,574 | 566,437
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 909 | 17,076
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 | 22,520 | 306,506
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 78 | 8,634
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 34,130 | 190,273
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 359 | 8,570
Debt Securities, Available-for-sale, Unrealized Loss Position | 56,650 | 496,779
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 437 | 17,204
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 | 281 | 1,161
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 | 3,061 | 5,163
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 14 | 60
Debt Securities, Available-for-sale, Unrealized Loss Position | 3,342 | 6,324
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 14 | 61
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 | 2,833 | 5,643
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 21 | 135
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 2,272 | 58,495
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 18 | 478
Debt Securities, Available-for-sale, Unrealized Loss Position | 5,105 | 64,138
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 39 | 613
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 | 4,500 | 34,852
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 62 | 793
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 | 4,500 | 34,852
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 62 | 793
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 | 83,555 | 123,848
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 441 | 1,727
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 | 83,555 | 123,848
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 441 | 1,727
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 | 4,394 | 13,813
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 13 | 33
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 5,167 | 77,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 42 | 591
Debt Securities, Available-for-sale, Unrealized Loss Position | 9,561 | 91,292
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 55 | $ 624
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 |
Collateral Pledged [Member] | Fixed Income Securities [Member] | US Government Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | $ 0 |

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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 + Details
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