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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 | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | $ 1,954,118 | $ 1,324,489
Less than 12 Months, Gross Unrealized Loss | 58,351 | 20,081
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 341,210 | 350,655
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 12,830 | 12,188
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 2,295,328 | 1,675,144
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 71,181 | 32,269
Short-Term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 18,822 | 8,803
Less than 12 Months, Gross Unrealized Loss | 4 | 4
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 Aggregate Losses Accumulated In Investments | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 18,822 | 8,803
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4 | 4
Fixed Income Investments And Other Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 1,954,118 | 1,260,222
Less than 12 Months, Gross Unrealized Loss | 58,351 | 19,970
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 341,210 | 350,655
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 12,830 | 12,188
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 2,295,328 | 1,610,877
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 71,181 | 32,158
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 1,935,296 | 1,251,419
Less than 12 Months, Gross Unrealized Loss | 58,347 | 19,966
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 341,210 | 350,655
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 12,830 | 12,188
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 2,276,506 | 1,602,074
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 71,177 | 32,154
Fixed Income Securities [Member] | Municipal Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 91,214 | 77,788
Less than 12 Months, Gross Unrealized Loss | 1,751 | 1,244
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 124,358 | 135,076
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 4,930 | 5,752
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 215,572 | 212,864
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 6,681 | 6,996
Fixed Income Securities [Member] | Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 677,529 | 453,504
Less than 12 Months, Gross Unrealized Loss | 15,731 | 4,998
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 104,242 | 172,045
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1,887 | 4,418
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 781,771 | 625,549
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 17,618 | 9,416
Fixed Income Securities [Member] | Foreign Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 33,646 | 20,827
Less than 12 Months, Gross Unrealized Loss | 1,389 | 748
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 7,842 | 14,277
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 893 | 596
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 41,488 | 35,104
Fixed Income Securities [Member] | U.S. Government Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 4,671 | 7,223
Less than 12 Months, Gross Unrealized Loss | 150 | 154
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 10,789 | 14,735
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 69 | 273
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 15,460 | 21,958
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 219 | 427
Fixed Income Securities [Member] | U.S. Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 0 | 25,039
Less than 12 Months, Gross Unrealized Loss | 0 | 2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 4,281 | 4,378
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 2 | 36
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 4,281 | 29,417
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 38
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 626,063 | 413,203
Less than 12 Months, Gross Unrealized Loss | 37,165 | 12,391
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 73,204 | 10,076
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 5,020 | 1,109
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 699,267 | 423,279
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 42,185 | 13,500
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 73,563 | 5,012
Less than 12 Months, Gross Unrealized Loss | 876 | 274
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 Aggregate Losses Accumulated In Investments | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 73,563 | 5,012
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 876 | 274
Fixed Income Securities [Member] | Other Asset-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 428,610 | 248,823
Less than 12 Months, Gross Unrealized Loss | 1,285 | 155
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 16,494 | 68
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 29 | 4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 445,104 | 248,891
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,314 | 159
Fixed Income Securities [Member] | Foreign Government Debt Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2,282 | 1,344
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 0 | 64,267
Less than 12 Months, Gross Unrealized Loss | 0 | 111
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 Aggregate Losses Accumulated In Investments | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 0 | 64,267
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 111
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member] | U.S. Government Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 0 | 64,267
Less than 12 Months, Gross Unrealized Loss | 0 | 111
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 | 0 | 64,267
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 111
Securities Pledged as Collateral [Member] | Securities Pledged as Collateral [Member] | U.S. Government Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | $ 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=27724398&loc=d3e27161-111563 + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563 + Details
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