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Financial Guarantee Insurance Contracts - Additional Information (Detail) £ in Millions, $ in Millions | 3 Months Ended | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2020 USD ($) | Sep. 30, 2020 USD ($) | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2020 GBP (£) | Jan. 01, 2020 USD ($) | Dec. 31, 2019 GBP (£) | Feb. 12, 2019 | Dec. 31, 2018 GBP (£) | Feb. 12, 2018 USD ($) | Dec. 31, 2017 USD ($)
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ceded Loss And Loss Expenses Paid Not Yet Recovered | $ 1 |  |  |  | $ 0 |  |  |  | $ 1 | $ 0 | $ 1 |  |  |  |  |  |  |
Net par amount of financial guarantees | 33,888 |  |  |  | 38,018 |  |  |  | 33,888 | 38,018 |  |  |  |  |  |  |  |
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest | 1,140 |  |  |  | $ 1,536 |  |  |  | 1,140 | $ 1,536 | 1,633 |  |  |  |  |  |  | $ 1,645
Stockholders Equity After Possible Increase in Domestic Public Finance Loss Reserves | (60) |  |  |  |  |  |  |  | (60) |  |  |  |  |  |  |  |  |
Possible Increase in Domestic Public Finance Loss Reserves | $ 1,200 |  |  |  |  |  |  |  | $ 1,200 |  |  |  |  |  |  |  |  |
Estimated Future Premium Payments Weighted Average Discounted Rate | 2.20% |  |  |  | 2.40% |  |  |  | 2.20% | 2.40% |  | 2.20% |  | 2.40% |  |  |  |
Financial Guarantee Insurance Contracts, Premium Receivable | $ 370 |  |  |  | $ 416 |  |  |  | $ 370 | $ 416 | 495 |  |  |  |  |  |  | 586
Amount Of Insured Par Outstanding Ceded To Reinsurer | 2,398 |  |  |  |  |  |  |  | $ 2,398 |  |  |  |  |  |  |  |  |
Ceded Principal Outstanding Major Reinsurer Percentage |  |  |  |  |  |  |  |  | 6.10% |  |  |  |  |  |  |  |  |
Deferred Ceded Premiums And Reinsurance Recoverables | 103 |  |  |  |  |  |  |  | $ 103 |  |  |  |  |  |  |  |  |
Reinsurance Payable | 27 |  |  |  | 29 |  |  |  | $ 27 | $ 29 |  |  |  |  |  |  |  |
Weighted average period of future premiums |  |  |  |  |  |  |  |  | 8 years 3 months 18 days | 8 years 6 months |  |  |  |  |  |  |  |
Uncollectable premium receivables | 17 |  |  |  |  |  |  |  | $ 17 |  |  |  |  |  |  |  |  |
Accelerated premium revenue for retired obligations |  |  |  |  |  |  |  |  | 12 | $ 10 | 32 |  |  |  |  |  |  |
Reinsurance recoveries of losses included in losses and loss expenses |  |  |  |  |  |  |  |  | (11) | (7) | (2) |  |  |  |  |  |  |
Losses and loss expense reserves ceded to reinsurers | 33 |  |  |  | 26 |  |  |  | 33 | 26 | 23 |  |  |  |  |  |  | $ 41
Subrogation recoveries, net of reinsurance | (1,725) |  |  |  | (1,702) |  |  |  | (1,725) | (1,702) |  |  |  |  |  |  |  |
Amortization of insurance intangible assets | 16 | $ 14 | $ 14 | $ 13 | 15 | $ 17 | $ 226 | $ 36 | 57 | 295 | 107 |  |  |  |  |  |  |
Intangible Assets, Gross (Excluding Goodwill) | 1,281 |  |  |  | 1,273 |  |  |  | 1,281 | 1,273 |  |  |  |  |  |  |  |
Finite-Lived Intangible Assets, Accumulated Amortization | 908 |  |  |  | 847 |  |  |  | 908 | 847 |  |  |  |  |  |  |  |
Intangible assets | 409 |  |  |  | 427 |  |  |  | 409 | 427 |  |  |  |  |  |  |  |
Letters of Credit Outstanding, Amount | 117 |  |  |  |  |  |  |  | 117 |  |  |  |  |  |  |  |  |
Financial Guarantee Outstanding Principal Ceded To Third Parties | 5,182 |  |  |  |  |  |  |  | 5,182 |  |  |  |  |  |  |  |  |
Public Finance Puerto Rico Net Par Outstanding | 1,070 |  |  |  |  |  |  |  | 1,070 |  |  |  |  |  |  |  |  |
Percent Cash & New COFINA Bonds Allocated Under POA |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 93.00% |  |  |
Ambac Assurance Remaining Policy Obligations | 80 |  |  |  |  |  |  |  | 80 |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 387 |  |  |  |  |  |  |  | 387 |  |  |  |  |  |  |  |  |
Allowance for Doubtful Accounts, Premiums and Other Receivables | 17 |  |  |  | 9 |  |  |  | 17 | 9 |  |  |  |  |  |  |  |
Premium Receivable, Credit Loss Expense (Reversal) |  |  |  |  |  |  |  |  | 9 |  |  |  |  |  |  |  |  |
Premium Receivable, Allowance for Credit Loss, Writeoff |  |  |  |  |  |  |  |  | (2) |  |  |  |  |  |  |  |  |
Premium Receivable, Allowance for Credit Loss, Recovery |  |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Financing Receivable, Past Due | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Financing Receivable, 90 Days or More Past Due, Still Accruing | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Reinsurance Recoverable, Allowance for Credit Loss | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
satisfaction and discharge of segregated account deferred amounts |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,000 |
satisfaction and discharge of segregated account deferred interest amounts |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 857 |
Loss and Loss Expense Benefit on Settled Deferred Amounts |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 288 |
Net Incurred RMBS Subrogation Recoveries |  |  |  |  |  |  |  |  | (23) | $ 42 | 62 |  |  |  |  |  |  |
Reinsurance Recoverable Credit Exposure | 1 |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |
Accounting Standards Update 2016-13 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | (4) |  |  |  |  |  |  |  | (4) |  |  |  |  |  |  |  |  |
Accounting Standards Update 2016-13 [Member] | Premiums Receivable [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  |  |  |  |  |  |  |  |  |  |  |  | $ (3) |  |  |  |  |
Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 302 |  |  |  |  |  |  |  | 302 |  |  |  |  |  |  |  |  |
IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 40 |  |  |  |  |  |  |  | 40 |  |  |  |  |  |  |  |  |
II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 3 |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |  |  |
III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 27 |  |  |  |  |  |  |  | 27 |  |  |  |  |  |  |  |  |
IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 15 |  |  |  |  |  |  |  | $ 15 |  |  |  |  |  |  |  |  |
Maximum [Member] | Furniture and Fixtures [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite-Lived Intangible Asset, Useful Life |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |
Loss Reserves [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average risk-free rate used to discount loss reserves |  |  |  |  |  |  |  |  | 1.10% | 2.10% |  |  |  |  |  |  |  |
United Kingdom, Pounds |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial Guarantee Insurance Contracts, Premium Receivable | 117 |  |  |  | 129 |  |  |  | $ 117 | $ 129 | $ 131 | £ 86 |  | £ 97 |  | £ 103 |  |
Commonwealth of Puerto Rico [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Domestic Public Finance Losses |  |  |  |  |  |  |  |  | 256 |  |  |  |  |  |  |  |  |
Public Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net par amount of financial guarantees | 15,497 |  |  |  | 17,653 |  |  |  | 15,497 | 17,653 |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 185 |  |  |  |  |  |  |  | 185 |  |  |  |  |  |  |  |  |
Public Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 157 |  |  |  |  |  |  |  | 157 |  |  |  |  |  |  |  |  |
Public Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 27 |  |  |  |  |  |  |  | 27 |  |  |  |  |  |  |  |  |
Public Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Public Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Public Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Structured Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net par amount of financial guarantees | 6,337 |  |  |  | 7,508 |  |  |  | 6,337 | 7,508 |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 59 |  |  |  |  |  |  |  | 59 |  |  |  |  |  |  |  |  |
Structured Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 27 |  |  |  |  |  |  |  | 27 |  |  |  |  |  |  |  |  |
Structured Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Structured Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 3 |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |  |  |
Structured Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 14 |  |  |  |  |  |  |  | 14 |  |  |  |  |  |  |  |  |
Structured Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 15 |  |  |  |  |  |  |  | 15 |  |  |  |  |  |  |  |  |
International Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net par amount of financial guarantees | 12,054 |  |  |  | 12,857 |  |  |  | 12,054 | 12,857 |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 144 |  |  |  |  |  |  |  | 144 |  |  |  |  |  |  |  |  |
International Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 118 |  |  |  |  |  |  |  | 118 |  |  |  |  |  |  |  |  |
International Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 13 |  |  |  |  |  |  |  | 13 |  |  |  |  |  |  |  |  |
International Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
International Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 13 |  |  |  |  |  |  |  | 13 |  |  |  |  |  |  |  |  |
International Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Housing Revenue [Member] | Public Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net par amount of financial guarantees | 5,855 |  |  |  | 5,991 |  |  |  | 5,855 | 5,991 |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 168 |  |  |  |  |  |  |  | 168 |  |  |  |  |  |  |  |  |
Housing Revenue [Member] | Public Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 155 |  |  |  |  |  |  |  | 155 |  |  |  |  |  |  |  |  |
Housing Revenue [Member] | Public Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 13 |  |  |  |  |  |  |  | 13 |  |  |  |  |  |  |  |  |
Housing Revenue [Member] | Public Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Housing Revenue [Member] | Public Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Housing Revenue [Member] | Public Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Housing Revenue [Member] | Military Housing [Member] | Public Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net par amount of financial guarantees | 5,575 |  |  |  | 5,654 |  |  |  | 5,575 | 5,654 |  |  |  |  |  |  |  |
Other Public Finance Sectors [Member] | Public Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 17 |  |  |  |  |  |  |  | 17 |  |  |  |  |  |  |  |  |
Other Public Finance Sectors [Member] | Public Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 2 |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |
Other Public Finance Sectors [Member] | Public Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 15 |  |  |  |  |  |  |  | 15 |  |  |  |  |  |  |  |  |
Other Public Finance Sectors [Member] | Public Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Other Public Finance Sectors [Member] | Public Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Other Public Finance Sectors [Member] | Public Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Mortgage-backed and Home Equity [Member] | Structured Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net par amount of financial guarantees | 3,635 |  |  |  | 4,423 |  |  |  | 3,635 | 4,423 |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 22 |  |  |  |  |  |  |  | 22 |  |  |  |  |  |  |  |  |
Mortgage-backed and Home Equity [Member] | Structured Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 3 |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |  |  |
Mortgage-backed and Home Equity [Member] | Structured Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Mortgage-backed and Home Equity [Member] | Structured Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 1 |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |
Mortgage-backed and Home Equity [Member] | Structured Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 3 |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |  |  |
Mortgage-backed and Home Equity [Member] | Structured Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 15 |  |  |  |  |  |  |  | 15 |  |  |  |  |  |  |  |  |
Structured Insurance [Member] | Structured Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net par amount of financial guarantees | 311 |  |  |  | 395 |  |  |  | 311 | 395 |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 14 |  |  |  |  |  |  |  | 14 |  |  |  |  |  |  |  |  |
Structured Insurance [Member] | Structured Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 14 |  |  |  |  |  |  |  | 14 |  |  |  |  |  |  |  |  |
Structured Insurance [Member] | Structured Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Structured Insurance [Member] | Structured Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Structured Insurance [Member] | Structured Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Structured Insurance [Member] | Structured Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Student Loan [Member] | Structured Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net par amount of financial guarantees | 626 |  |  |  | 769 |  |  |  | 626 | 769 |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 16 |  |  |  |  |  |  |  | 16 |  |  |  |  |  |  |  |  |
Student Loan [Member] | Structured Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 3 |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |  |  |
Student Loan [Member] | Structured Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Student Loan [Member] | Structured Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 2 |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |
Student Loan [Member] | Structured Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 11 |  |  |  |  |  |  |  | 11 |  |  |  |  |  |  |  |  |
Student Loan [Member] | Structured Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Other Structured Finance [Member] | Structured Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 7 |  |  |  |  |  |  |  | 7 |  |  |  |  |  |  |  |  |
Other Structured Finance [Member] | Structured Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 7 |  |  |  |  |  |  |  | 7 |  |  |  |  |  |  |  |  |
Other Structured Finance [Member] | Structured Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Other Structured Finance [Member] | Structured Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Other Structured Finance [Member] | Structured Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Other Structured Finance [Member] | Structured Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Sovereign/Sub-sovereign [Member] | International Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net par amount of financial guarantees | 5,270 |  |  |  | 5,264 |  |  |  | 5,270 | 5,264 |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 108 |  |  |  |  |  |  |  | 108 |  |  |  |  |  |  |  |  |
Sovereign/Sub-sovereign [Member] | International Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 82 |  |  |  |  |  |  |  | 82 |  |  |  |  |  |  |  |  |
Sovereign/Sub-sovereign [Member] | International Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 13 |  |  |  |  |  |  |  | 13 |  |  |  |  |  |  |  |  |
Sovereign/Sub-sovereign [Member] | International Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Sovereign/Sub-sovereign [Member] | International Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 13 |  |  |  |  |  |  |  | 13 |  |  |  |  |  |  |  |  |
Sovereign/Sub-sovereign [Member] | International Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Investor-owned and Public Utilities [Member] | International Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net par amount of financial guarantees | 3,899 |  |  |  | $ 4,436 |  |  |  | 3,899 | $ 4,436 |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 31 |  |  |  |  |  |  |  | 31 |  |  |  |  |  |  |  |  |
Investor-owned and Public Utilities [Member] | International Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 31 |  |  |  |  |  |  |  | 31 |  |  |  |  |  |  |  |  |
Investor-owned and Public Utilities [Member] | International Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Investor-owned and Public Utilities [Member] | International Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Investor-owned and Public Utilities [Member] | International Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Investor-owned and Public Utilities [Member] | International Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Other International [Member] | International Finance [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 5 |  |  |  |  |  |  |  | 5 |  |  |  |  |  |  |  |  |
Other International [Member] | International Finance [Member] | Surveillance Category One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 5 |  |  |  |  |  |  |  | 5 |  |  |  |  |  |  |  |  |
Other International [Member] | International Finance [Member] | IA [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Other International [Member] | International Finance [Member] | II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Other International [Member] | International Finance [Member] | III [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | 0 |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |
Other International [Member] | International Finance [Member] | IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Premiums Receivable, Gross | $ 0 |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |

X
- Definition Ambac Assurance Remaining Policy Obligations + References No definition available. + Details
Name: | ambc_AmbacAssuranceRemainingPolicyObligations
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of insured par outstanding ceded to reinsurer. + References No definition available. + Details
Name: | ambc_AmountOfInsuredParOutstandingCededToReinsurer
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition This element represents the reduction to gross claim liability for subrogation on insured RMBS financial obligations with credit deterioration, net of reinsurance. + References No definition available. + Details
Name: | ambc_BalanceOfRmbsSubrogationRecoveriesNetOfReinsurance
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Ceded Loss And Loss Expenses Paid Not Yet Recovered + References No definition available. + Details
Name: | ambc_CededLossAndLossExpensesPaidNotYetRecovered
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Ceded Principal Outstanding Major Reinsurer Percentage. + References No definition available. + Details
Name: | ambc_CededPrincipalOutstandingMajorReinsurerPercentage
Namespace Prefix: | ambc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Includes the unexpired portion of financial guarantee premiums ceded on policies as of the balance sheet date and reinsurance recoverables on claims or claim adjustment expense paid by the ceding insurer. + References No definition available. + Details
Name: | ambc_DeferredCededPremiumsAndReinsuranceRecoverables
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Domestic Public Finance Losses + References No definition available. + Details
Name: | ambc_DomesticPublicFinanceLosses
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Weighted average discounted rate of estimated future premium payments to be paid by the VIEs. + References No definition available. + Details
Name: | ambc_EstimatedFuturePremiumPaymentsWeightedAverageDiscountedRate
Namespace Prefix: | ambc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Financial Guarantee Outstanding Principal Ceded to third parties. + References No definition available. + Details
Name: | ambc_FinancialGuaranteeOutstandingPrincipalCededToThirdParties
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Insurance [Line Items] + References No definition available. + Details
Name: | ambc_InsuranceLineItems
Namespace Prefix: | ambc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The known and estimated amount recoverable as of the balance sheet date from reinsurers for claims expected to be paid or incurred by the ceding insurer. + References No definition available. + Details
Name: | ambc_LossReservesCededToReinsurers
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Loss and Loss Expense Benefit on Settled Deferred Amounts + References No definition available. + Details
Name: | ambc_LossandLossExpenseBenefitonSettledDeferredAmounts
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Net Incurred RMBS Subrogation Recoveries + References No definition available. + Details
Name: | ambc_NetIncurredRMBSSubrogationRecoveries
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percent Cash & New COFINA Bonds Allocated Under POA + References No definition available. + Details
Name: | ambc_PercentCashNewCOFINABondsAllocatedUnderPOA
Namespace Prefix: | ambc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Possible Increase in Domestic Public Finance Loss Reserves + References No definition available. + Details
Name: | ambc_PossibleIncreaseinDomesticPublicFinanceLossReserves
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Public Finance Puerto Rico Net Par Outstanding + References No definition available. + Details
Name: | ambc_PublicFinancePuertoRicoNetParOutstanding
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Reinsurance Recoverable Credit Exposure + References No definition available. + Details
Name: | ambc_ReinsuranceRecoverableCreditExposure
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition satisfaction and discharge of segregated account deferred amounts + References No definition available. + Details
Name: | ambc_Satisfactionanddischargeofsegregatedaccountdeferredamounts
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition satisfaction and discharge of segregated account deferred interest amounts + References No definition available. + Details
Name: | ambc_Satisfactionanddischargeofsegregatedaccountdeferredinterestamounts
Namespace Prefix: | ambc_
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- Definition The valuation allowance as of the balance sheet date to reduce the gross amount of receivables to estimated net realizable value, which would be presented in parentheses on the face of the balance sheet. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5074-111524 + Details
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