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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, 2016 | Dec. 31, 2015
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | $ 990,704 | $ 2,281,230
Less than 12 Months, Gross Unrealized Loss | 13,911 | 82,799
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 695,623 | 463,320
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 38,746 | 21,407
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 1,686,327 | 2,744,550
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 52,657 | 104,206
Short-Term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 0 | 9,982
Less than 12 Months, Gross Unrealized Loss | 0 | 1
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 | 9,982
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 1
Fixed Income Investments And Other Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 990,704 | 2,216,675
Less than 12 Months, Gross Unrealized Loss | 13,911 | 82,742
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 695,623 | 463,320
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 38,746 | 21,407
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 1,686,327 | 2,679,995
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 52,657 | 104,149
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 990,704 | 2,206,693
Less than 12 Months, Gross Unrealized Loss | 13,911 | 82,741
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 695,623 | 463,320
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 38,746 | 21,407
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 1,686,327 | 2,670,013
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 52,657 | 104,148
Fixed Income Securities [Member] | Municipal Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 35,192 | 117,008
Less than 12 Months, Gross Unrealized Loss | 1,410 | 2,070
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 103,482 | 114,708
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 3,341 | 6,118
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 138,674 | 231,716
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 4,751 | 8,188
Fixed Income Securities [Member] | Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 201,291 | 938,916
Less than 12 Months, Gross Unrealized Loss | 1,611 | 21,331
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 9,937 | 92,581
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 50 | 3,001
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 211,228 | 1,031,497
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,661 | 24,332
Fixed Income Securities [Member] | U.S. Government Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 1,883 | 2,938
Less than 12 Months, Gross Unrealized Loss | 18 | 18
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 5,070 | 10,658
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 5 | 170
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 6,953 | 13,596
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 23 | 188
Fixed Income Securities [Member] | U.S. Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 0 | 0
Less than 12 Months, Gross Unrealized Loss | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 4,106 | 4,212
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 2 | 27
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 4,106 | 4,212
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 27
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 388,104 | 584,699
Less than 12 Months, Gross Unrealized Loss | 10,199 | 53,367
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 517,756 | 213,303
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 35,089 | 11,250
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 905,860 | 798,002
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 45,288 | 64,617
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 7,409 | 77,538
Less than 12 Months, Gross Unrealized Loss | 26 | 1,481
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 29,459 | 0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 233 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 36,868 | 77,538
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 259 | 1,481
Fixed Income Securities [Member] | Other Asset-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 354,221 | 450,690
Less than 12 Months, Gross Unrealized Loss | 641 | 3,456
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 25,813 | 19,274
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 26 | 36
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 380,034 | 469,964
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 667 | 3,492
Fixed Income Securities [Member] | Foreign Government Debt Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 2,604 | 34,904
Less than 12 Months, Gross Unrealized Loss | 6 | 1,018
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0 | 8,584
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 0 | 805
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 2,604 | 43,488
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 6 | 1,823
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 0 | 64,555
Less than 12 Months, Gross Unrealized Loss | 0 | 57
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 | 64,555
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 57
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 | 64,555
Less than 12 Months, Gross Unrealized Loss | 0 | 57
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 | 64,555
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 0 | $ 57

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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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- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + 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)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563 + 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 less than twelve months. + 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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