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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 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | $ 1,988,778 | $ 1,491,493
Less than 12 Months, Gross Unrealized Loss | 86,480 | 29,944
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 633,662 | 788,764
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 21,563 | 49,698
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 2,622,440 | 2,280,257
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 108,043 | 79,642
Short-Term [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 251,926 | 65,176
Less than 12 Months, Gross Unrealized Loss | 209 | 44
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 | 251,926 | 65,176
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 209 | 44
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 | 108,043 | 79,642
Fixed Income Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 1,736,852 | 1,426,317
Less than 12 Months, Gross Unrealized Loss | 86,271 | 29,900
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 633,662 | 788,764
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 21,563 | 49,698
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 2,370,514 | 2,215,081
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 107,834 | 79,598
Fixed Income Securities [Member] | Municipal Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 667,335 | 98,147
Less than 12 Months, Gross Unrealized Loss | 68,578 | 2,045
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 32,525 | 122,928
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 822 | 5,160
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 699,860 | 221,075
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 69,400 | 7,205
Fixed Income Securities [Member] | Corporate Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 292,028 | 963,513
Less than 12 Months, Gross Unrealized Loss | 3,377 | 20,232
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 87,272 | 6,492
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 2,094 | 328
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 379,300 | 970,005
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 5,471 | 20,560
Fixed Income Securities [Member] | Foreign Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 8,122 | 5,063
Less than 12 Months, Gross Unrealized Loss | 81 | 100
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 1,700 | 0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 30 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 9,822 | 5,063
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 111 | 100
Fixed Income Securities [Member] | U.S. Government Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 74,188 | 6,037
Less than 12 Months, Gross Unrealized Loss | 1,653 | 93
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 5,525 | 5,045
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 100 | 4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 79,713 | 11,082
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1,753 | 97
Fixed Income Securities [Member] | U.S. Agency Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value |  | 4,060
Less than 12 Months, Gross Unrealized Loss |  | 3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | 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 |  | 4,060
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | 3
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 668,524 | 226,889
Less than 12 Months, Gross Unrealized Loss | 12,524 | 7,201
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 418,617 | 550,807
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 17,958 | 36,584
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 1,087,141 | 777,696
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 30,482 | 43,785
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value |  | 6,986
Less than 12 Months, Gross Unrealized Loss |  | 23
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | 25,780
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments |  | 197
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 32,766
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 220
Fixed Income Securities [Member] | Other Asset-Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than 12 Months, Fair Value | 26,655 | 115,622
Less than 12 Months, Gross Unrealized Loss | 58 | 203
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 88,023 | 77,712
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 559 | 7,425
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 114,678 | 193,334
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 617 | 7,628
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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- 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=82897076&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=82897076&loc=d3e27337-111563 + Details
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