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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
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Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | $ 401 | $ 1,139
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 4 | 27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 88 | 360
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1 | 11
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 489 | 1,499
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 5 | 38
Below Investment Grade Securities and Non-Rated Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Fair Value | 29 | 660
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses | 0 | 18
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 201 | 115
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 | 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 | 201 | 115
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Fixed Income Investments And Other Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 5 |
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 200 | 1,024
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 4 | 27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 88 | 360
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1 | 11
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 288 | 1,384
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 5 | 38
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 | 13 | 538
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 16
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 10 | 29
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 0 | 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 23 | 566
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 17
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 | 36 | 6
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 | 2 | 58
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 | 38 | 64
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 1
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 | 63 | 307
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 2 | 9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 5 | 190
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 0 | 9
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 68 | 497
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 17
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 | 20 | 1
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 | 5
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 | 20 | 6
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
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 | 5 | 35
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 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 | 5 | 35
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 1
Fixed Income Securities [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 7 |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 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 | 7 |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 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 | 53 | 124
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 63 | 0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 1 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 116 | 124
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 | 2
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 | 2 | 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 | 7 | 77
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments | 0 | 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 10 | 91
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | $ 1
Collateral Pledged [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 |
Collateral Pledged [Member] | Fixed Income Securities [Member] | Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | $ 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=118939692&loc=d3e27161-111563 + Details
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