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Background and Basis of Presentation - Surplus Note Exchange - Ambac Assurance Corporation [Member] | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 USD ($) | Feb. 11, 2021 USD ($) | Jan. 19, 2021 USD ($) | Dec. 31, 2019 USD ($)
Insurance [Line Items] |  |  |  |
Long-term Debt, Gross | $ 2,884,000,000 |  |  | $ 2,980,000,000
5.1 % Surplus Notes Due 2020 [Member] |  |  |  |
Insurance [Line Items] |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 5.10% |  |  | 5.10%
5.1% Junior Surplus Notes Due 2020 [Member] |  |  |  |
Insurance [Line Items] |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 5.10% |  |  | 5.10%
Corolla Notes | 5.1 % Surplus Notes Due 2020 [Member] |  |  |  |
Insurance [Line Items] |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  | 5.10% |
Lease Reducing Notes | 5.1% Junior Surplus Notes Due 2020 [Member] |  |  |  |
Insurance [Line Items] |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 5.10% |  |
5.1% Junior Surplus Notes, General Account Due 2020 [Member] |  |  |  |
Insurance [Line Items] |  |  |  |
Long-term Debt, Gross | $ 365,000,000 |  |  | $ 365,000,000
5.1% Junior Surplus Notes, General Account Due 2020 [Member] | Corolla Notes |  |  |  |
Insurance [Line Items] |  |  |  |
One Dollar Outstanding |  |  | $ 1.00 |
Payment Amount in Exchange |  |  | 0.9125 |
Note Holders | 1 |  |  |
5.1% Junior Surplus Notes, General Account Due 2020 [Member] | Corolla Certificate |  |  |  |
Insurance [Line Items] |  |  |  |
One Dollar Outstanding |  |  | 1.00 |
Payment Amount in Exchange |  |  | 0.64 |
5.1% Junior Surplus Notes, General Account Due 2020 [Member] | Lease Reducing Notes |  |  |  |
Insurance [Line Items] |  |  |  |
One Dollar Outstanding |  | $ 1.00 |  |
Payment Amount in Exchange |  | 0.8581 |  |
5.1% Junior Surplus Notes, General Account Due 2020 [Member] | Lease Reducing Notes | Lease Reducing Notes |  |  |  |
Insurance [Line Items] |  |  |  |
Long-term Debt Reduced in Exchange |  | 15,000,000 |  |
5.1% Surplus Notes, General Account, Due 2020 [Member] |  |  |  |
Insurance [Line Items] |  |  |  |
Long-term Debt, Gross | $ 531,000,000 |  |  | $ 531,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member] | Corolla Notes |  |  |  |
Insurance [Line Items] |  |  |  |
Long-term Debt Issued in Exchange Transaction |  |  | 267,000,000 |
5.1% Surplus Notes, General Account, Due 2020 [Member] | Lease Reducing Notes |  |  |  |
Insurance [Line Items] |  |  |  |
Long-term Debt Issued in Exchange Transaction |  |  | $ 13,000,000 |
5.1% Surplus Notes, General Account, Due 2020 [Member] | Corolla Notes & Junior Surplus Notes |  |  |  |
Insurance [Line Items] |  |  |  |
Long-term Debt, Gross |  | 853,000,000 |  |
Long-term Debt, Gross and Accrued and Unpaid Interest |  | 1,414,000,000 |  |
5.1% Surplus Notes, General Account, Due 2020 [Member] | Corolla Notes & Junior Surplus Notes | Parent Company [Member] |  |  |  |
Insurance [Line Items] |  |  |  |
Long-term Debt, Gross |  | $ 83,000,000 |  |

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