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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020
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Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | $ 477 | $ 816
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 9 | 7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 25 | 25
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 503 | 841
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 9 | 8
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 77 | 187
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 7 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 84 | 187
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Fixed Income Investments And Other Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 9 | 8
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 401 | 629
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 8 | 7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 19 | 25
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 419 | 654
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 9 | 8
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 | 42 | 25
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 2 | 6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 43 | 31
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1 | 0
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 | 243 | 543
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 5 | 6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 12 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 256 | 543
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 6 | 6
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 | 78 | 3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 2 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 78 | 3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 2 | 0
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 | 19 | 17
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1 | 1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 19 | 17
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1 | 1
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 | 0 | 14
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 2 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 2 | 14
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
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 | 18 | 27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 3 | 15
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 21 | 42
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 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 | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0 | 4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position | 0 | 4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Collateral Pledged [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Collateral Pledged [Member] | Fixed Income Securities [Member] | Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | $ 0 | $ 0

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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=121645371&loc=d3e27161-111563 + Details
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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922890-210455 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27337-111563 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456 + Details
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