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Background and Business Description - Additional Information (Details) - USD ($) | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2021 | Feb. 11, 2021 | Jan. 19, 2021
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Entity Incorporation, Date of Incorporation | Apr. 29, 1991 |  |  |  |
Net realized gains (losses) on extinguishment of debt | $ 33,000,000 |  |  |  |
Realized Investment Gains (Losses) | 2,000,000 | $ 8,000,000 |  |  |
Business Acquisition, Percentage of Voting Interests Acquired |  |  | 80.00% |  |
Parent |  |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Realized Investment Gains (Losses) | $ 4,000,000 |  |  |  |
Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes, General Account Due 2020 |  |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Note Holders | 1 |  |  |  |
One Dollar Outstanding |  |  |  | $ 1.00 | $ 1.00
Payment Amount in Exchange |  |  |  | 0.8581 |
Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes, General Account Due 2020 | Corolla Notes |  |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Payment Amount in Exchange |  |  |  |  | 0.9125
Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes, General Account Due 2020 | Corolla Certificate |  |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Payment Amount in Exchange |  |  |  |  | 0.6400
Ambac Assurance Corporation [Member] | 5.1% Surplus Notes, General Account, Due 2020 |  |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Long-term Debt, Gross |  |  |  | 853,000,000 |
Long-term Debt, Gross and Accrued and Unpaid Interest |  |  |  | 1,414,000,000 |
Ambac Assurance Corporation [Member] | 5.1% Surplus Notes, General Account, Due 2020 | Corolla Notes |  |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Long-term Debt Issued in Exchange Transaction |  |  |  |  | 267,000,000
Ambac Assurance Corporation [Member] | 5.1% Surplus Notes, General Account, Due 2020 | Lease Reducing Notes |  |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Long-term Debt Issued in Exchange Transaction |  |  |  |  | $ 13,000,000
Ambac Assurance Corporation [Member] | 5.1% Surplus Notes, General Account, Due 2020 | Parent Company [Member] |  |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Long-term Debt, Gross |  |  |  | 83,000,000 |
Ambac Assurance Corporation [Member] | 5.1 % Surplus Notes Due 2020 |  |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  | 5.10%
Ambac Assurance Corporation [Member] | Lease Reducing Notes | 5.1% Junior Surplus Notes, General Account Due 2020 |  |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Long-term Debt Reduced in Exchange |  |  |  | $ 15,000,000 |
Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes Due 2020 |  |  |  |  |
Background And Basis Of Presentation [Line Items] |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  | 5.10% |

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