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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | $ 1,139,101 | $ 1,988,778
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 27,298 | 86,480
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 359,943 | 633,662
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 10,897 | 21,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 1,499,044 | 2,622,440
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 38,195 | 108,043
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 | 660,063 | 1,855,694
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses | 17,838 | 100,503
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 115,374 | 251,926
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 97 | 209
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 115,374 | 251,926
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 97 | 209
Fixed Income Investments And Other Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | 108,043
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 1,023,727 | 1,736,852
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 27,201 | 86,271
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 359,943 | 633,662
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 10,897 | 21,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 1,383,670 | 2,370,514
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 38,098 | 107,834
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 | 537,904 | 667,335
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 15,878 | 68,578
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 28,533 | 32,525
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1,198 | 822
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 566,437 | 699,860
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 17,076 | 69,400
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 | 5,643 | 74,188
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 135 | 1,653
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 58,495 | 5,525
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 478 | 100
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 64,138 | 79,713
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 613 | 1,753
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 | 306,506 | 292,028
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 8,634 | 3,377
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 190,273 | 87,272
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 8,570 | 2,094
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 496,779 | 379,300
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 17,204 | 5,471
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 | 1,161 | 8,122
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1 | 81
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 5,163 | 1,700
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 60 | 30
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 6,324 | 9,822
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 61 | 111
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 | 34,852 | 668,524
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 793 | 12,524
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 418,617
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 0 | 17,958
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 34,852 | 1,087,141
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 793 | 30,482
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 | 123,848 |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1,727 |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 |
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 123,848 |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,727 | 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 | 13,813 | 26,655
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 33 | 58
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 77,479 | 88,023
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 591 | 559
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 91,292 | 114,678
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 624 | 617
Collateral Pledged [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | 0
Collateral Pledged [Member] | Fixed Income Securities [Member] | US Government Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | $ 0

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- Definition Aggregate losses of below investment grade securities and non rated securities in a continuous unrealized loss position. + References No definition available. + Details
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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=82897076&loc=d3e27161-111563 + Details
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