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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - Successor [Member] - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | $ 642,107 | $ 1,988,778
Less than 12 Months, Gross Unrealized Loss | 17,304 | 86,480
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 265,430 | 633,662
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 17,469 | 21,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 907,537 | 2,622,440
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 34,773 | 108,043
Below Investment Grade Securities and Non-Rated Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Fair Value | 176,712 | 1,855,694
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses | 12,293 | 100,503
Short-Term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 71,044 | 251,926
Less than 12 Months, Gross Unrealized Loss | 75 | 209
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 71,044 | 251,926
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 75 | 209
Fixed Income Investments And Other Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 34,773 | 108,043
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 571,063 | 1,736,852
Less than 12 Months, Gross Unrealized Loss | 17,229 | 86,271
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 265,430 | 633,662
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 17,469 | 21,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 836,493 | 2,370,514
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 34,698 | 107,834
Fixed Income Securities [Member] | Municipal Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 34,650 | 667,335
Less than 12 Months, Gross Unrealized Loss | 244 | 68,578
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 29,761 | 32,525
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1,557 | 822
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 64,411 | 699,860
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,801 | 69,400
Fixed Income Securities [Member] | Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 433,313 | 292,028
Less than 12 Months, Gross Unrealized Loss | 14,673 | 3,377
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 75,649 | 87,272
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 4,681 | 2,094
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 508,962 | 379,300
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 19,354 | 5,471
Fixed Income Securities [Member] | U.S. Government Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 65,033 | 74,188
Less than 12 Months, Gross Unrealized Loss | 1,714 | 1,653
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 3,914 | 5,525
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 81 | 100
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 68,947 | 79,713
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,795 | 1,753
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 7,107 | 668,524
Less than 12 Months, Gross Unrealized Loss | 394 | 12,524
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 76,481 | 418,617
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 6,990 | 17,958
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 83,588 | 1,087,141
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 7,384 | 30,482
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Fixed Income Securities [Member] | Other Asset-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 17,759 | 26,655
Less than 12 Months, Gross Unrealized Loss | 70 | 58
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 77,994 | 88,023
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 4,120 | 559
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 95,753 | 114,678
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4,190 | 617
Fixed Income Securities [Member] | Foreign Government Debt Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 13,201 | 8,122
Less than 12 Months, Gross Unrealized Loss | 134 | 81
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 1,631 | 1,700
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 40 | 30
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 14,832 | 9,822
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 174 | 111
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member] | U.S. Government Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 0 | $ 0

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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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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