Basis of Presentation and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Operating Leased Assets [Line Items]
Consolidation	Consolidation The consolidated financial statements include the accounts of AFG and all other entities in which AFG (directly or through its subsidiaries) has a controlling financial interest, including variable interest entities (“VIEs”) for which AFG or an AFG subsidiary is deemed the primary beneficiary in accordance with the Consolidation Topic of the Accounting Standards Codification ("ASC"). All significant intercompany balances have been eliminated. The usual condition for a controlling financial interest is ownership of a majority of the voting interests of an entity. However, a controlling financial interest may also exist in entities, such as VIEs, through arrangements that do not involve controlling voting interests. A VIE is an entity: a) that lacks enough equity investment at risk to permit the entity to finance its activities without additional subordinated financial support from other parties; or b) where the group of equity holders does not have: (1) the power, through voting rights or similar rights, to direct the activities of an entity that most significantly impact the entity’s economic performance; (2) the obligation to absorb the entity’s expected losses; or (3) the right to receive the entity’s expected residual returns. The determination of whether a variable interest holder is the primary beneficiary involves performing a qualitative analysis of the VIE that includes, among other factors, its capital structure, contractual terms including the rights of each variable interest holder, the activities of the VIE, whether the variable interest holder has the power to direct the activities of a VIE that most significantly impact the VIE’s economic performance, whether the variable interest holder has the obligation to absorb losses of the VIE that could potentially be significant to the VIE or the right to receive benefits from the VIE that could potentially be significant to the VIE, related party relationships and the design of the VIE. An entity that is deemed the primary beneficiary of a VIE is required to consolidate the VIE. Refer to Note 4. Variable Interest Entities, for a detailed discussion of Ambac’s involvement in VIEs, Ambac’s methodology for determining whether Ambac is required to consolidate a VIE and the effects of VIEs being consolidated.
Unconsolidated Financial Information	AFG Unconsolidated Financial InformationFinancial information of AFG is presented in Schedule II to this Form 10-K as of December 31, 2020 and 2019 and for the years ended December 31, 2020, 2019 and 2018. Investments in subsidiaries are accounted for using the equity method of accounting in Schedule II.
Financing Receivable, Allowance for Credit Losses, Policy for Uncollectible Amounts	Measurement of Credit Losses on Financial Instruments (CECL) On January 1, 2020 Ambac adopted ASU 2016-13, Financial Instruments-Credit Losses (Topic 326) - Measurement of Credit Losses on Financial Instruments, subsequently amended by ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments - Credit Losses; ASU 2019-04, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments; ASU 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief; and ASU 2019-11, Codification Improvements to Topic 326, Financial Instruments - Credit Losses (collectively the Current Expected Credit Loss standard or "CECL"). The new CECL standard affects how reporting entities measure credit losses for financial assets that are not accounted for at fair value through net income. For Ambac, these financial assets include available-for-sale debt securities and amortized cost assets, specifically premium receivables, reinsurance recoverables and loans. CECL does not apply to recoveries of previously paid losses on financial guarantee insurance contracts accounted for under ASC 944 nor does it apply to equity method investments accounted for under ASC 323. •For available-for-sale debt securities, credit losses under CECL are measured similarly to other-than-temporary impairments under prior GAAP. The updated guidance was applied prospectively. •For financial instruments measured at amortized cost, CECL replaces the "incurred loss" model, which generally delayed recognition of the full amount of credit losses until the loss was probable of occurring, with an "expected loss" model, which reflects an entity's current estimate of all expected lifetime credit losses. The estimate of expected lifetime credit losses should consider historical information, current information, as well as reasonable and supportable forecasts. Expected lifetime credit losses for amortized cost assets will be recorded as an allowance for credit losses, with subsequent increases or decreases in the allowance reflected in net income each period. The updated guidance was applied by a cumulative effect adjustment to the opening balance of retained earnings at January 1, 2020. This adjustment was not material to retained earnings or any individual balance sheet line item. Refer to the discussion below for each asset type. As a result of adopting CECL, management revised its policies and procedures around the credit impairment evaluation process. CECL also introduced new disclosures related to the credit impairment process, including certain accounting policy elections that Ambac made under the new standard.
Investments	Investments The Investments - Debt Securities Topic of the ASC requires that all debt instruments be classified in Ambac’s Consolidated Balance Sheets according to their purpose and, depending on that classification, be carried at either cost or fair market value. Ambac’s non-VIE debt investment portfolio is accounted for on a trade-date basis and consists primarily of investments in fixed maturity securities that are considered available-for-sale as defined by the Investments - Debt Securities Topic of the ASC. Available-for-sale debt securities are reported in the financial statements at fair value with unrealized gains and losses, net of deferred taxes, reflected in Accumulated Other Comprehensive Income (Loss) in Stockholders’ Equity and computed using amortized cost as the basis. For purposes of computing amortized cost, premiums and discounts are accounted for using the effective interest method over a future term of the security. For structured debt securities with a large underlying pool of homogenous loans, such as mortgage-backed and asset-backed securities, premiums and discounts are adjusted for the effects of actual and anticipated prepayments. For other fixed maturity securities, such as corporate and municipal bonds, discounts were amortized or accreted over the remaining term of the securities. Ambac adopted ASU 2017-08, Receivables-Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities, on January 1, 2019. ASU 2017-08 shortened the amortization period for the premium on callable debt securities to the earliest call date. Under previous GAAP, Ambac generally amortized the premium over the contractual life (i.e. maturity) of the debt security and if that debt security was called, we would record a loss equal to the unamortized premium. Ambac’s non-VIE investment portfolio also includes equity interests in pooled investment funds which are accounted for in accordance with the Investments - Equity Securities Topic of the ASC and reported as Other investments on the Consolidated Balance Sheet with income reported through Net investment income on the Statement of Total Comprehensive Income (Loss). Equity interests in the form of common stock or in-substance common stock are classified as trading securities and reported at fair value while limited partner interests in such funds are reported using the equity method. Fair value is based primarily on quotes obtained from independent market sources. When quotes for fixed maturity securities are not available or cannot be reasonably corroborated, valuation models are used to estimate fair value. These models include estimates, made by management, which utilize current market information. When fair value is not readily determinable for pooled investment funds, the investments are valued using net asset value ("NAV") as a practical expedient as permitted under the Fair Value Measurement Topic of the ASC. Investment valuations could differ materially from amounts that would actually be realized in the market. Realized gains and losses on the sale of investments are determined on the basis of specific identification. VIE investments in fixed maturity securities are carried at fair value as they are either considered as available for sale securities or under the fair value option election. For additional information about VIE investments, including fair value by asset-type, see Note 4. Variable Interest Entities. Ambac has a formal credit impairment review process for fixed maturity available-for-sale securities in its investment portfolio. Ambac conducts a review each quarter to identify and evaluate investments that have indications of impairment in accordance with the Investments - Debt Securities Topic of the ASC. •Prior to the adoption of CECL, factors considered to identify and assess securities for other than temporary impairment include: (i) fair values that have declined by 20% or more below amortized cost; (ii) market values that have declined by 5% or more but less than 20% below amortized cost for a continuous period of at least six months; (iii) recent downgrades by rating agencies; (iv) the financial condition of the issuer and financial guarantor, as applicable, and an analysis of projected defaults on the underlying collateral; (v) whether scheduled interest payments are past due; (vi) whether Ambac has the intent to sell the security; and (vii) whether it is more likely than not that Ambac will be required to sell a security before the anticipated recovery of its amortized cost basis. If we believed a decline in the fair value of a particular investment is not credit-related, we recorded the decline as an unrealized loss net of tax in Accumulated Other Comprehensive Income (Loss) in Stockholders’ Equity on our Consolidated Balance Sheets. If it was determined that a credit impairment existed, the credit impairment loss was recognized in earnings, and the other-than-temporary amount related to all other factors was recognized in other comprehensive income. For fixed maturity securities that have credit impairments in a period, the previous amortized cost of the security less the amount of the credit impairment recorded through earnings becomes the investment’s new amortized cost basis. Ambac accretes the new amortized cost basis to par or to the estimated future cash flows to be recovered over the expected remaining life of the security. •Under CECL, credit losses are evaluated and measured similarly, however the recognition of credit impairment losses for available-for-sale debt securities are recorded as an allowance for credit losses with an offsetting charge to net income, rather than as a direct write-down of the security as was required under prior GAAP. As a result, improvements to estimated credit losses for available-for-sale debt securities are recognized immediately in net income rather than as interest income over time. Furthermore, as required under CECL, Ambac no longer considers the length of time a security has continuously been in an unrealized loss in the credit impairment process. If we believe a decline in the fair value of a particular investment is not credit impaired, we record the decline as an unrealized loss net of tax in Accumulated Other Comprehensive Income (Loss) in Stockholders’ Equity on our Consolidated Balance Sheets. If management either: (i) has the intent to sell its investment in a debt security or (ii) determines that the Company more likely than not will be required to sell the debt security before its anticipated recovery of the amortized cost basis less any current period credit impairment, then an impairment charge is recognized in earnings, with the amortized cost of the security being written-down to fair value. The evaluation of securities for credit impairment is a quantitative and qualitative process, which is subject to risks and uncertainties and is intended to determine whether, and to what extent, declines in the fair value of investments should be recognized in current period earnings. The risks and uncertainties include changes in general economic conditions, the issuer’s or guarantor’s financial condition and/or future prospects, the impact of regulatory actions on the investment portfolio, the performance of the underlying collateral, the effects of changes in interest rates or credit spreads and the expected recovery period. With respect to Ambac insured securities owned, future cash flows used to measure credit impairment represents the sum of (i) the bond’s intrinsic cash flows and (ii) the estimated AAC claim payments. Ambac’s assessment about whether a decline in value is considered a credit impairment reflects management’s current judgment regarding facts and circumstances specific to a security and the factors noted above. If that judgment changes, Ambac may ultimately record a charge for other-than-temporary impairment in future periods. Ambac has made certain accounting policy elections related to accrued interest receivable ("AIR") for available-for-sale investments under CECL, which are consistent with past practices under prior GAAP. Elections include: i) not measuring AIR for credit impairment, instead AIR is written off when it becomes 90 days past due; ii) writing off AIR by reversing interest income; iii) presenting AIR separately in Other Assets on the balance sheet and iv) excluding AIR from amortized cost balances in required CECL disclosures found in Note 11. Investments. AIR at December 31, 2020 was $10. Refer to Note 11. Investments for further credit impairment disclosures.
Net Premiums Earned	Net Premiums Gross premiums were received either upfront or in installments. For premiums received upfront, an unearned premium revenue (“UPR”) liability was established, which was initially recorded as the cash amount received. For installment premium transactions, a premium receivable asset and offsetting UPR liability was initially established in an amount equal to: (i) the present value of future contractual premiums due (the “contractual” method) or (ii) if the underlying insured obligation is a homogenous pool of assets which are contractually prepayable, the present value of premiums to be collected over the expected life of the transaction (the “expected” method). An appropriate risk-free rate corresponding to the weighted average life of each policy and currency is used to discount the future premiums contractually due or expected to be collected. For example, U.S. dollar exposures are discounted using U.S. Treasury rates while exposures denominated in a foreign currency are discounted using the appropriate risk-free rate for the respective currency. The weighted average risk-free rate at December 31, 2020 and 2019, was 2.2%. and 2.4%, respectively, and the weighted average period of future premiums used to estimate the premium receivable at December 31, 2020 and 2019, was 8.3 years and 8.5 years, respectively. Insured obligations consisting of homogeneous pools for which Ambac uses expected future premiums to estimate the premium receivable include residential mortgage-backed securities ("RMBS"). As prepayment assumptions change for homogenous pool transactions, or if there is an actual prepayment for a “contractual” method installment transaction, the related premium receivable and UPR are adjusted in equal and offsetting amounts with no immediate effect on earnings using new premium cash flows and the then current risk-free rate corresponding to the initial weighted average life of the related policy. For both upfront and installment premium policies, premium revenues are earned over the life of the financial guarantee contract in proportion to the insured principal amount outstanding at each reporting date (referred to as the level-yield method). For installment paying policies, the premium receivable discount, equating to the difference between the undiscounted future installment premiums and the present value of future installment premiums, is accreted as premiums earned in proportion to the premium receivable balance at each reporting date. •For financial guarantee contracts, the issuer's ability and willingness to pay its insured debt obligation impacts the payment of policy losses by Ambac as well as the receipt of premiums from the issuer. As such, management leverages its existing loss reserve estimation process to evaluate credit impairment for premium receivables. Key factors in assessing credit impairment include historical premium collection data, internal risk classifications, credit ratings and loss severities. For structured finance transactions involving special purpose entities, we further evaluate the priority of premiums paid to Ambac within the contractual waterfall, as required by bond indentures. Ambac has a formal quarterly credit impairment review process for premium receivables under financial guarantee insurance contracts. •Prior to the adoption of CECL, Ambac assessed collectability of premium receivables in accordance with ASC 944 and recorded an allowance for uncollectible premiums. •Under CECL, management utilizes either a discounted cash flow ("DCF") or probability of default/loss given default ("PD/LGD") approach to estimate credit impairment. The DCF approach utilizes expected cash flows developed by Ambac's Risk Management Group using the same (or similar) models used for estimating loss reserves where such models can identify shortfalls in premiums. Credit impairment using the DCF approach is equal to the difference between amortized cost and the present value of expected cash flows. Credit impairment under the PD/LGD approach is the product of (i) the premium receivable carrying value, (ii) internally developed default probability (considering internal ratings and average life), and (iii) internally developed loss severities. Refer to Note 8. Financial Guarantee Insurance Contracts for further credit impairment disclosures. AAC has reinsurance in place pursuant to surplus share treaty and facultative reinsurance agreements. Similar to gross premiums, premiums ceded to reinsurers were paid either upfront or in installments. For premiums paid upfront, a deferred ceded premium asset was established which is initially recorded as the cash amount paid. For installment premiums, a ceded premiums payable liability and offsetting deferred ceded premium asset were initially established in an amount equal to: i) the present value of future contractual premiums due or ii) if the underlying insured obligation is a homogenous pool of assets, the present value of expected premiums to be paid over the life of the transaction. An appropriate risk-free rate corresponding to the weighted average life of each policy and exposure currency is used to discount the future premiums contractually due or expected to be collected. Premiums ceded to reinsurers reduce the amount of premiums earned by Ambac from its financial guarantee insurance policies. For both upfront and installment premiums, ceded premiums written are primarily recognized in earnings in proportion to and at the same time as the related gross premium revenue is recognized. For premiums paid to reinsurers on an installment basis, Ambac records the present value of future ceding commissions as an offset to ceded premiums payable, using the same assumptions noted above for installment premiums. When a bond issue insured by Ambac has been retired early, typically due to an issuer call, any remaining UPR is recognized at that time to the extent the financial guarantee contract is legally extinguished, causing accelerated premium revenue. For installment premium paying transactions, we offset the recognition of any remaining UPR by the reduction of the related premium receivable to zero (as it will not be collected as a result of the retirement), which may cause negative accelerated premium revenue. Certain obligations insured by Ambac have been legally defeased whereby government securities are purchased by the issuer with the proceeds of a new bond issuance, or less frequently with other funds of the issuer, and held in escrow. The principal and interest received from the escrowed securities are then used to retire the Ambac-insured obligations at a future date either to their maturity date (a refunding) or a specified call date (a pre-refunding). Ambac has evaluated the provisions in policies issued on these obligations and determined those insurance policies have not been legally extinguished. For policies with refunding securities, premium revenue recognition is not impacted as the escrowed maturity date is the same as the previous legal maturity date. For policies with pre-refunding securities, the maturity date of the pre-refunded security has been shortened from its previous legal maturity. Although premium revenue recognition has not been accelerated in the period of the pre-refunding, it results in an increase in the rate at which the policy's remaining UPR is to be recognized.
Loans	Loans Loans are reported at either their outstanding principal balance less unamortized discount or at fair value. •Loans not held by consolidated VIEs are reported at their outstanding principal balance less unamortized discount and are reported within Other assets on the Consolidated Balance Sheet. Interest income is earned using the effective interest method based upon interest accrued on the unpaid principal balance adjusted for accretion of discounts. A loan is considered impaired when, based on the financial condition of the borrower, it is probable that Ambac will be unable to collect all principal and interest due according to the contractual terms of the loan agreement. Under CECL, Ambac has a formal quarterly credit impairment review process for these loans. The key factors in assessing credit impairment are internal credit ratings and loss severities. Management utilizes a PD/LGD approach, similar to the one described above for premium receivables, which is applied to the loan carrying value. •Loans held by VIEs consolidated as required under the Consolidation Topic of the ASC are carried at fair value under the fair value option election with changes in fair value recorded in Income (loss) on variable interest entities on the Consolidated Statements of Total Comprehensive Income (Loss). Such loans are reported as Loans, at fair value within the Variable interest entity assets section of the Consolidated Balance Sheet.
Derivative Contracts	Derivative Contracts The Company has entered into derivative contracts to hedge certain economic risks inherent in its asset and liability portfolios. None of Ambac’s derivative contracts are designated as hedges under the Derivatives and Hedging Topic of the ASC. Ambac's derivatives consist primarily of interest rate swaps and futures contracts. •Ambac maintains a portfolio consisting primarily of interest rate swaps and futures contracts to economically hedge interest rate risk in the financial guarantee and investment portfolios. While this portfolio also includes certain legacy interest rate swaps executed in connection with financial guarantee client financings, the interest rate derivatives portfolio is managed on the basis of its net sensitivity to changes in interest rates. Changes in the fair value of these interest rate derivatives are recorded, along with changes in fair value of Ambac's remaining credit derivatives, within Net gains (losses) on derivative contracts on the Consolidated Statements of Total Comprehensive Income (Loss). •VIEs consolidated under the Consolidation Topic of the ASC entered into derivative contracts to meet specified purposes within their securitization structure. Changes in fair value of consolidated VIE derivatives are included within Income (loss) on variable interest entities on the Consolidated Statements of Total Comprehensive Income (Loss). All derivatives are recorded on the Consolidated Balance Sheets at fair value on a gross basis; assets and liabilities are netted by counterparty only when a legal right of offset exists. Variation payments on centrally cleared swaps and futures contracts are considered settlements of the associated derivative balances and are reflected as a reduction to derivative liabilities or assets on the Consolidated Balance Sheets. For other derivatives, Ambac has determined that the amounts recognized for the right to reclaim cash collateral or the obligation to return cash collateral may not be used to offset amounts due under the derivative instruments in the normal course of settlement. Therefore, such amounts are not offset against fair value amounts recognized for derivative instruments executed with the same counterparty under the same master netting arrangement and are included in "Other assets" on the Consolidated Balance Sheets. Refer to Note 12. Derivative Instruments for further discussion of the Company’s use of derivative instruments and their impact of the consolidated financial statements. Refer to Note 10. Fair Value Measurements for further description of the methodologies used to determine the fair value of derivative contracts, including model inputs and assumptions where applicable.
Restricted Cash	Restricted Cash Cash that we do not have the right to use for general purposes is recorded as restricted cash in our consolidated balance sheets. Restricted cash includes (i) consolidated variable interest entity cash restricted to support the obligations of the consolidated VIEs, (ii) cash held by AAC received from its investment in Secured Notes and pledged for the benefit of holders of Secured Notes (other than AAC) and (iii) fiduciary cash held by Xchange described below. Fiduciary Assets and Liabilities: In Xchange's capacity as a managing general agent, generally it collects premiums from insureds and remits the premiums to the respective insurance carriers, net of fees to other parties, including its commissions. Xchange also collects claims or refunds from carriers on behalf of insureds. Unremitted insurance premiums and claims proceeds are held by Xchange in a fiduciary capacity. Since fiduciary assets are not available for corporate use, they are shown in the consolidated balance sheets as an offset to fiduciary liabilities, which are reported in Other liabilities.Restricted cash for net uncollected premiums and claims and the related fiduciary liabilities were $4 at December 31, 2020.
Loss Reserves	Loss and Loss Expenses The loss and loss expense reserve (“loss reserve”) policy relates only to Ambac’s non-derivative insurance business for insurance policies issued to beneficiaries, including VIEs, for which we do not consolidate the VIE. Losses and loss expenses are based upon estimates of the ultimate aggregate losses inherent in the non-derivative financial guarantee portfolio as of the reporting date. The policy for derivative contracts is discussed in the “Derivative Contracts” section above. A loss reserve is recorded on the balance sheet on a policy-by-policy basis based upon the present value ("PV") of expected net claim cash outflows or expected net recovery cash inflows, discounted at risk-free rates. The estimate for future net cash flows consider the likelihood of all possible outcomes that may occur from missed principal and/or interest payments on the insured obligation. This estimate also considers future recoveries related to breaches of contractual representations and warranties by RMBS transaction sponsors, remediation strategies, excess spread and other contractual or subrogation-related cash flows. Ambac’s approach to resolving disputes involving contractual breaches by transaction sponsors or other third parties has included negotiations and/or pursuing litigation. Ambac does not estimate recoveries for litigations where its sole claim is for fraudulent inducement, since any remedies under such claims would be non-contractual. •Net claim cash outflow policies represent contracts where the PV of expected cash outflows are greater than the PV of expected recovery cash inflows. For such policies, a “Loss and loss expense reserves” liability is recorded for the excess of the PV of expected net claim cash outflows over the unearned premium revenue. •Net recovery cash inflow policies represent contracts where the PV of expected recovery cash inflows are greater than the PV of expected claim cash outflows. For such policies, a “Subrogation recoverable” asset is recorded. The evaluation process for determining expected losses is subject to certain judgments based on our assumptions regarding the probability of default by the issuer of the insured security, probability of settlement outcomes (which may include commutation settlements, refinancing and/or other settlement outcomes) and expected severity of credits for each insurance contract. Ambac’s loss reserves are based on management’s ongoing review of the financial guarantee credit portfolio. Active surveillance of the insured portfolio enables Ambac’s Risk Management Group ("RMG") to track credit migration of insured obligations from period to period and update internal classifications and credit ratings for each transaction. Non- adversely classified credits are assigned a Class I rating while adversely classified credits are assigned a rating of Class IA through Class V. The criteria for an exposure to be assigned an adversely classified credit rating includes the deterioration of an issuer’s financial condition, underperformance of the underlying collateral (for collateral dependent transactions such as mortgage-backed or student loan securitizations), poor performance by the servicer of the underlying collateral and other adverse economic events or trends. The servicer of the underlying collateral of an insured securitization transaction is a consideration in assessing credit quality because the servicer’s performance can directly impact the performance of the related issue. For example, a servicer of a mortgage-backed securitization that does not remain current in its collection efforts could cause an increase in the delinquency and the potential for default of the underlying obligation. Similarly, loss severities increase when a servicer does not effectively handle loss mitigation activities such as (i) the advancing of delinquent principal and interest and of default related expenses which are deemed to be recoverable by the servicer, (ii) pursuit of loan charge-offs which maximize cash flows from the mortgage loan pool and (iii) foreclosure and real estate owned disposition strategies and timelines. All credits are assigned risk classifications by RMG using the following guidelines: CLASS I - “Fully Performing - Meets Ambac Criteria with Remote Probability of Claim” - Credits that demonstrate adequate security and structural protection with a strong capacity to pay interest, repay principal and perform as underwritten. Factors supporting debt service payment and performance are considered unlikely to change and any such change would not have a negative impact upon the fundamental credit quality. Through ongoing surveillance, Ambac may also designate Class I credits into one or more of the following categories: •Survey List - credits that may lack information or demonstrate a weakness but further deterioration is not expected. •Watch List - credits that demonstrate the potential for future material adverse development due to such factors as long-term uncertainty about a particular sector, a certain structural element or concern related to the issuer or transaction or the overall financial and economic sustainability. CLASS IA - “Potential Problem with Risks to be Dimensioned” - Credits that are fully current and monetary default or claims-payment are not anticipated. The payor’s or issuer’s financial condition may be deteriorating or the credits may lack adequate collateral. A structured financing may also evidence weakness in its fundamental credit quality as evidenced by its under-performance relative to its modeled projections at underwriting, issues related to the servicer’s ability to perform or questions about the structural integrity of the transaction. While certain of these credits may still retain an investment grade rating, they usually have experienced or are vulnerable to a ratings downgrade. Further investigation is required to dimension and correct any deficiencies. A complete legal review of documents may be required. An action plan should be developed with triggers for future classification changes upward or downward. CLASS II - “Substandard Requiring Intervention” - Credits whose fundamental credit quality has deteriorated to the point that timely payment of debt service may be jeopardized by adversely developing trends of a financial, economic, structural, managerial or political nature. No claim payment is currently foreseen but the probability of loss or claim payment over the life of the transaction is now existent (generally 10% or greater probability). Class II credits may be border-line or below investment grade (BBB- to B). Prompt and sustained action must be taken to execute a comprehensive loss mitigation plan and correct deficiencies. CLASS III - “Doubtful with Clear Potential for Loss” - Credits whose fundamental credit quality has deteriorated to the point that timely payment of debt service has been or will be jeopardized by adverse trends of a financial, economic, structural, managerial or political nature which, in the absence of positive change or corrective action, are likely to result in a loss. The probability of monetary default or claims paying over the life of the transaction is generally 50% or greater. Full exercise of all available remedial actions is required to avert or minimize losses. Class III credits will generally be rated below investment grade (B to CCC). CLASS IV - “Imminent Default or Defaulted” - Monetary default or claim payments have occurred or are expected imminently. Class IV credits are generally rated D. CLASS V - “Fully Reserved” - The credit has defaulted and payments have occurred. The claim payments are scheduled and known, reserves have been established to fully cover such claims, and no claim volatility is expected. The population of credits evaluated in Ambac’s loss reserve process are: (i) all adversely classified credits (Class IA through V) and ii) non-adversely classified credits which had an internal Ambac rating downgrade since the transaction’s inception. One of two approaches is then utilized to estimate losses to ultimately determine if a loss reserve should be established. The first approach is a statistical expected loss approach, which considers the likelihood of all possible outcomes. The “base case” statistical expected loss is the product of: (i) the par outstanding on the credit; (ii) internally developed default information (taking into consideration internal ratings and average life of an obligation); (iii) internally developed loss severities; and (iv) a discount factor. The loss severities and default information are based on rating agency information, are specific to each bond type and are established and approved by senior RMG officers. For certain credit exposures, Ambac’s additional monitoring, loss remediation efforts and probabilities of potential settlement outcomes may provide information relevant to adjust this estimate of “base case” statistical expected losses. Analysts may accept the “base case” statistical expected loss as the best estimate of expected loss or assign multiple probability weighted scenarios to determine an adjusted statistical expected loss that better reflects management’s view of a given transaction’s expected losses, as well as the potential for additional remediation activities (e.g., commutations). The second approach entails the use of cash-flow based models to estimate expected losses (future claims, net of potential recoveries, expected to be paid to the holder of the insured financial obligation). Ambac’s RMG group will consider the likelihood of all possible outcomes and develop appropriate cash flow scenarios. This approach can include the utilization of internal or third party models and tools to project future losses and resultant claim payment estimates. We utilize cash flow models for RMBS, student loan, Puerto Rico and other exposures. RMBS and student loan models use historical performance of the collateral pools in order to then derive future performance characteristics, such as default and voluntary prepayment rates, which in turn determine projected future claim payments. In other cases, such as many public finance exposures, including our Puerto Rico exposures, we do not specifically forecast resources available to pay debt service in the cash flow model itself. Rather, we consider the issuers’ overall ability and willingness to pay, including the fiscal, economic, legal and political framework. In this approach, a probability-weighted expected loss estimate is developed based on assigning probabilities to multiple claim payment scenarios and applying an appropriate discount factor. Additionally, we assign a probability to the issuer’s ability to refinance an insured issue, Ambac’s ability to execute a potential settlement (i.e., commutation) of the insurance policy, including the impact on future installment premiums, and/or other restructuring scenarios. The commutation scenarios and the related probabilities of occurrence vary by transaction, depending on our view of the likelihood of negotiating such a transaction with issuers and/or investors. The discount factor applied to the statistical expected loss approach is based on a risk-free discount rate corresponding to the remaining expected weighted-average life of the exposure and the exposure currency. For the cash flow scenario approach, discount factors are applied based on a risk-free discount rate term structure and correspond to the date of each respective cash flow payment or recovery and the exposure currency. Discount factors are updated for the current risk-free rate each reporting period. Ambac establishes loss expense reserves based on our estimate of expected net cash outflows for loss expenses, such as legal and consulting costs. Below we provide further details of our loss reserve models for both RMBS and student loan exposures: RMBS Expected Loss Estimate Ambac insures RMBS transactions collateralized by first-lien mortgages. Ambac has also insured RMBS transactions collateralized predominantly by second-lien mortgage loans such as closed-end seconds and home equity lines of credit. A second-lien mortgage loan is a type of loan in which the borrower uses the equity in their home as collateral and the second-lien loan is subordinate to the first-lien loan outstanding on the home. Borrowers are obligated to make monthly payments on both their first and second-lien loans. If the borrower defaults on the payments due under these loans and the property is subsequently liquidated, the liquidation proceeds are first utilized to pay off the first-lien loan (as well as other costs) and any remaining funds are applied to pay off the second-lien loan. As a result of this subordinate position to the first-lien loan, second-lien loans may carry a significantly higher severity in the event of a loss, approaching or exceeding 100%. Ambac primarily utilizes a cash flow model (“RMBS cash flow model”) to develop estimates of projected losses for both our first and second lien transactions. First, the RMBS cash flow model projects collateral performance utilizing: (i) the transaction’s underlying loans' characteristics and status, (ii) projected home price appreciation (“HPA”) and (iii) projected interest rates. Depending in the amount of collateral information available for each transaction, we project such performance either at the loan-level or the deal-level. In the absence of specific loan-level information, the deal-level approach evaluates a loan pool as if it were a single loan, selecting certain aggregated deal-level characteristics to then perform a series of statistical analyses. The deal-level approach projects performance using a roll-rate that evaluates the possible future state of a loan based on its current status and three variables: average FICO (credit score), average current consolidated loan to value ratio (“CLTV”) and an overall quality indicator. Observed servicer-level behavior may also have an impact on projected transaction performance. We source HPA projections from a market accepted vendor and interest rate projections are developed from market sources. We use three HPA projection scenarios to develop a base case as well as stress and upside cases. The highest probability is assigned to the base case, with lower probabilities to the stress and upside cases. For the liabilities of the transaction which we insure, we generally utilize waterfall projections generated from a tool provided by a market accepted vendor. This waterfall tool allows us to capture the impact of each transaction’s specific structure (e.g., the waterfall priority of payments, triggers, redemption priority) to generate our specific projected claims profile in the base, upside and downside scenarios. On a monthly basis, we compare monthly claims submitted against the trustees’ reports, waterfall projections and our understanding of the transactions’ structures to identify and resolve discrepancies. We also review the vendor’s published waterfall revisions to identify significant discrepancies. Resolving discrepancies is challenging and may take place over an extended period of time. Moreover, transaction documents are subject to interpretation, and our interpretation or that of the vendor and as reflected in our loss reserves may prove to be incorrect and/or not consistent with trustees directing cash flows in the future. In some cases, we may utilize an alternative waterfall structure when our legal and commercial analysis of the transaction’s payment structure differs from the vendor’s waterfall structure. In our experience, market performance and model characteristics change and therefore need to be updated and reflected in our models through time. As such, we conduct regular reviews of current models, alternative models and the overall approach to loss estimation. RMBS Representation and Warranty Subrogation Recoveries Ambac records, as a component of its loss reserve estimate, subrogation recoveries related to securitized loans in RMBS transactions that breached certain representations and warranties described herein. Generally, the sponsor of an RMBS transaction provided representations and warranties with respect to the securitized loans, including representations and warranties with respect to loan characteristics, the absence of borrower fraud in the underlying loan pools and other misconduct in the origination process and attesting to the compliance of loans with the prevailing underwriting policies. In such cases, the sponsor of the transaction is contractually obligated to repurchase, cure or substitute collateral for any loan that breaches the representations or warranties. Ambac or its counsel engaged consultants with significant mortgage underwriting experience to review the underwriting documentation for mortgage loans underlying certain insured RMBS transactions which exhibited exceptionally poor performance. Factors which Ambac believes to be indicative of poor performance include (i) high levels of early payment defaults, (ii) significant numbers of loan liquidations or charge-offs and resulting high levels of losses and (iii) rapid elimination of credit protections inherent in the transactions’ structures. With respect to item (ii), “loan liquidations” refers to loans for which the servicer has liquidated the related collateral and the securitization has realized losses on the loan; “charge-offs” refers to loans which have been written off as uncollectible by the servicer, generating no recoveries to the securitization, and may also refer to the unrecovered balance of liquidated loans. In either case, the servicer has taken actions to recover against the collateral, and the securitization has incurred losses to the extent such actions did not result in full repayment of the borrower’s obligations. Generally, subsequent to the forensic exercise of examining loan files to ascertain whether the loans conformed to the representations and warranties, we submitted nonconforming loans for repurchase to the contractual counterparty bearing the repurchase obligation, typically the transaction sponsor. In cases where loans are repurchased by a sponsor, the effect is typically to offset current period losses and then to increase the over-collateralization of the securitization, depending on the extent of loan repurchases and the structure of the securitization. Specifically, the repurchase price is paid by the sponsor to the securitization trust which holds the loan. The cash becomes an asset of the trust, replacing the loan that was repurchased by the sponsor. On a monthly basis, the cash received related to loan repurchases by the sponsor is aggregated with cash collections from the underlying mortgages and applied in accordance with the trust indenture payment waterfall. To the extent there continues to be insufficient cash in the waterfall in the current month to make scheduled principal and interest payments to the note holders, Ambac is required to make additional claim payments to cover this shortfall. Ambac may also receive payments directly from transaction sponsors in settlement of their repurchase obligations pursuant to negotiated settlement agreements or otherwise as a result of related litigation. While the obligation by sponsors to repurchase loans with material breaches is clear, generally the sponsors have not honored those obligations without actual or threatened litigation. Ambac has utilized the results of the above described loan file examinations to make demands for loan repurchases from sponsors or their successors and, in certain instances, as a part of the basis for litigation. Ambac’s approach to resolving these disputes has included negotiating with individual sponsors at the transaction level and in some cases at the individual loan level and has resulted in the repurchase of some loans. Ambac has initiated and will continue to pursue lawsuits seeking compliance with the repurchase obligations in the securitization documents. Ambac has performed the above-mentioned, detailed examinations on a variety of second-lien and first-lien transactions that have experienced exceptionally poor performance. However, the loan file examinations and related estimated recoveries we have reviewed and recorded to date have been limited to only those transactions whose sponsors (or their successors) are subsidiaries of large financial institutions, all of which carry an investment grade rating from at least one nationally recognized rating agency, or are otherwise deemed to have the financial wherewithal to live up to their repurchase obligations. While our contractual recourse is generally to the sponsor/subsidiary, rather than to the parent, each of these large institutions has significant financial resources and may have an ongoing interest in mortgage finance, and we therefore believe that the financial institution/parent would ultimately assume financial responsibility for these obligations if the sponsor/subsidiary is unable to honor its contractual obligations or pay a judgment that we may obtain in litigation. Additionally, in the case of successor institutions, we are not aware of any provisions that explicitly preclude or limit the successors’ ability to honor the obligations of the original sponsor. Certain successor financial institutions have made significant payments to certain claimants to settle breaches of representations and warranties perpetrated by sponsors that have been acquired by such financial institutions. For example, Ambac received a significant payment in 2016 from JP Morgan to settle RMBS-related litigation. As a result of these factors, we do not make significant adjustments to our estimated subrogation recoveries with respect to the credit risk of these sponsors or their successors. Our ability to realize RMBS R&W subrogation recoveries is subject to significant uncertainty, including risks inherent in litigation; collectability of such amounts from counterparties (and/or their respective parents and affiliates); timing of receipt of any such recoveries; intervention by OCI, which could impede our ability to take actions required to realize such recoveries; and uncertainty inherent in the assumptions used in estimating such recoveries. Failure to realize RMBS R&W subrogation recoveries for any reason or the realization of RMBS R&W subrogation recoveries materially below the amount recorded on Ambac's consolidated balance sheet would have a material adverse effect on our results of operations and financial condition and may result in adverse consequences such as impairing the ability of AAC to honor its financial obligations; the initiation of rehabilitation proceedings against AAC; decreased likelihood of AAC delivering value to Ambac, through dividends or otherwise; and a significant drop in the value of securities issued and/or insured by Ambac or AAC. The approach used to estimate RMBS R&W subrogation recoveries is based on obtaining loan files from the original pool and conducting loan file re-underwriting to derive a breach rate to be extrapolated to determine an estimated repurchase obligation. We limit the estimated repurchase obligation by ever-to-date incurred losses. Multiple probability-weighted scenarios are developed by applying various realization factors to the estimated repurchase obligation. The realization factors in these scenarios reflect Ambac’s own assumptions about the likelihood of outcomes based on all the information available to it including, but not limited to, (i) discussions with external legal counsel and their views on ultimate settlement and/or litigation outcomes, (ii) assessment of the strength of the specific case and (iii) experience in settling similar claims. The probability weightings are developed based on the unique facts and circumstances for each transaction. The sum of these probability-weighted scenarios represents the undiscounted RMBS R&W subrogation recovery, which is then discounted using a factor derived from a risk-free discount rate term structure that corresponds to the estimated date of each respective recovery. Student Loan Expected Loss Estimate The student loan insured portfolio consists of credits collateralized by private student loans. The calculation of loss reserves for our student loan portfolio involves evaluating numerous factors that can impact ultimate losses. The factor which contributes the greatest degree of uncertainty in ascertaining appropriate loss reserves is the long final legal maturity date of the insured bonds. Most of the student loan bonds which we insure were issued with original terms of 20 to 40 years until final maturity. Since our policy covers timely interest and ultimate principal payment, our loss projections must make assumptions for many factors covering a long time horizon. Key assumptions that will impact ultimate losses include, but are not limited to, the following: collateral performance (which is highly correlated to the economic environment); interest rates; operating risks associated with the issuer, servicers, special servicers, and administrators; investor appetite for tendering or commuting insured obligations and; as applicable, Ambac’s ability and willingness to commute policies. In addition, we consider in our student loan loss projections the potential impact, if any, of proposed or final regulatory actions or orders, including by the Consumer Financial Protection Bureau ("CFPB"), affecting our insured transactions. In evaluating our student loan portfolio, our losses are projected using a cash flow modeling approach. In order to project collateral performance under the cash flow approach, we use a default projection tool that constructs lifetime cohort default curves based on loan and deal-level historical performance data. To determine ultimate losses on the transactions, the cohort default curves are used to extrapolate future default behavior. Additionally, a regression-based model is used to estimate recoveries on defaulted loans. This regression-based recovery forecast is grounded in deal-level performance data. For the liabilities of the transaction which we insure, the transaction losses are then incorporated into a waterfall tool to develop loss estimates for our exposures in various base, upside and downside scenarios. We develop and assign probabilities to multiple cash flow scenarios based on each transaction’s unique characteristics. Probabilities assigned are based on available data related to the credit, information from contact with the issuer (if applicable), and any economic or market information that may impact the outcomes of the various scenarios being evaluated. Our base case usually projects deal performance out to maturity using expected loss assumptions. As appropriate, we also develop other cases that incorporate various upside and downside scenarios that may include changes to defaults and recoveries.
Reinsurance Accounting Policy	Ceded Reinsurance Loss and loss expense reserve reported on the balance sheet relates only to direct insurance policies. The corresponding reserve ceded to reinsurers is reported as reinsurance recoverable on paid and unpaid losses. AAC has reinsurance in place pursuant to surplus share treaty and facultative reinsurance agreements. The reinsurance of risk does not relieve AAC of its original liability to its policyholders. In the event that any of Ambac Assurance’s reinsurers are unable to meet their obligations under reinsurance contracts, AAC would, nonetheless, be liable to its policyholders for the full amount of its policy. Credit exposure exists with respect to reinsurance recoverables to the extent that any reinsurer may not be able to reimburse AAC under the terms of these reinsurance arrangements. To minimize its exposure to losses from reinsurers, AAC (i) monitors the financial condition of its reinsurers; (ii) is entitled to receive collateral from its reinsurance counterparties in certain reinsurance contracts; and (iii) has certain cancellation rights that can be exercised by AAC in the event of rating agency downgrades of a reinsurer (among other events and circumstances). Under CECL, Ambac has a formal quarterly credit impairment review process whereby Ambac has elected to use the practical expedient of considering the fair value of collateral posted by reinsurers when evaluating credit impairment. To determine the total unsecured recoverable to be evaluated for credit impairment, Ambac nets the reinsurance recoverable amount by ceded premiums payable and the fair value of collateral posted, if any. The key factors in assessing credit impairment for reinsurance recoverables are independent rating agency credit ratings and loss severities. Management utilizes a PD/LGD approach, similar to the one described above for premium receivables, which is applied to the net unsecured reinsurance recoverable amount. Refer to Note 8. Financial Guarantee Insurance Contracts for further credit impairment disclosures.
Debt, Policy	Long-Term Debt Long-term debt issued by Ambac is carried at par value less unamortized discount. Accrued interest and discount accretion on long-term debt is reported as Interest expense on the Consolidated Statements of Total Comprehensive Income (Loss). To the extent Ambac repurchases or redeems its long- term debt, such repurchases or redemptions may be settled for an amount different than the carrying value of the obligation. Any difference between the payment and carrying value of the obligation is reported in Net realized gains (losses) on extinguishment of debt on the Consolidated Statements of Total Comprehensive Income (Loss). For long-term debt issued by consolidated VIEs in which Ambac's variable interest arises from financial guarantees written by Ambac's subsidiaries ("FG VIEs"), we may elect to use the fair value option on an instrument by instrument basis. When the fair value option is elected, changes in the fair value of the FG VIEs' long-term debt is reported within Income (loss) on variable interest entities in the Consolidated Statements of Total Comprehensive Income (Loss), except for the portion of the total change in fair value of financial liabilities caused by changes in the instrument-specific credit risk which is presented separately in Other comprehensive income (loss). In cases where the fair value option has not been elected, the FG VIEs' long-term debt is carried at par less unamortized discount, with interest expense reported within Income (loss) on variable interest entities in the Consolidated Statements of Total Comprehensive Income (Loss).
Noncontrolling Interest Policy [Policy Text Block]	Noncontrolling Interests Nonredeemable noncontrolling interests At December 31, 2020 and 2019, AAC had 5,501 shares of issued and outstanding AMPS with a liquidation preference of $138 (reported as nonredeemable noncontrolling interest of $60 on Ambac's balance sheet). The auction occurs every 28 days and the dividend rate has continuously been reset at the maximum rate of one-month LIBOR plus 200 basis points. Under the terms of the AMPS, dividends may not be paid on the common stock of AAC unless all accrued and unpaid dividends on the AMPS for the then current dividend period have been paid, provided, that dividends on the common stock may be made at all times for the purpose of, and only in such amounts as are necessary for, enabling AFG (i) to service its indebtedness for borrowed money as such payments become due or (ii) to pay its operating expenses. If dividends are paid on the common stock as provided in the prior sentence, dividends on the AMPS become cumulative until the date that all accumulated and unpaid dividends have been paid on the AMPS. AAC has not paid dividends on its AMPS since 2010. Redeemable noncontrolling interests The Xchange acquisition, further described in Note 3. Business Combination, resulted in 80% ownership of the acquired entities by Ambac. Under the terms of the acquisition agreement, Ambac has a call option to purchase the remaining 20% from the minority owners (i.e., noncontrolling interests) and the minority owners have a put option to sell the remaining 20% to Ambac. The call and put options are exercisable after different time periods elapse. Because the exercise of the put option is outside the control of Ambac, in accordance with the Distinguishing Liabilities from Equity Topic of the ASC, Ambac reports redeemable noncontrolling interests in the mezzanine section of its consolidated balance sheet.
Postretirement and Postemployment Benefits	Postretirement and Postemployment BenefitsAmbac provides postretirement and postemployment benefits, including health and life benefits covering employees who meet certain age and service requirements. Ambac accounts for these benefits under the accrual method of accounting. Amounts related to the postretirement health benefits liability are established and charged to expense based on actuarial determinations.
Long Term Incentive and Stock Compensation Plan	Incentive Compensation Incentive compensation is a key component of our compensation strategy. Incentive compensation has two components: short term incentive compensation (consisting of an annual cash bonus and, prior to 2020, awards of deferred stock units for certain officers) and long term incentive plan awards (consisting of deferred cash and awards of restricted and performance stock units). Annual decisions with regard to incentive compensation are generally made in the first quarter of each year and are based on the prior year's performance for the Company, the employee and the employee's business unit. In 2020, the Ambac 2013 Incentive Compensation Plan (the “2013 Incentive Plan”) was superseded by the 2020 Incentive Compensation Plan ("2020 Incentive Plan"). Both plans allow for the granting of stock options, restricted stock, stock appreciation rights, restricted and performance units and other awards to employees, directors and consultants that are valued or determined by reference to Ambac's common stock. Under these plans, Ambac has issued both cash and equity awards to US employees. In connection with the adoption of the 2020 Incentive Plan, all shares reserved but unissued under the 2013 Incentive Plan were transferred to the the 2020 Incentive Plan in addition to any shares underlying outstanding awards under the 2013 Incentive Plan as of June 2, 2020 that subsequently terminate by expiration or forfeiture, cancellation, or otherwise are not issued. Under the 2013 and 2020 Incentive Compensation Plans. Ambac recognizes compensation costs for all equity classified awards granted at fair value, which is measured on the grant date, and records forfeitures for unvested shares only when they occur. For awards that only include service and performance conditions, the fair value is the market price of Ambac stock on the grant date. For awards that also contain a market condition, specifically a total shareholder return ("TSR") modifier, the fair value is estimated using a Monte Carlo simulation. The types of equity awards granted to employees are as follows: •Deferred stock units granted vest upon grant and will settle and convert to Ambac common stock annually over a two-year period (50% on the first anniversary of the grant date and 50% on the second anniversary of the grant date). The fair value of these grants is recognized as compensation expense on the date of grant since no future service is required. •Restricted stock units granted only require future service and accordingly the respective fair value is recognized as compensation expense over the relevant service period. •Performance stock units granted require both future service and achieving specified performance targets to vest. Certain performance stock unit grants also include a market condition TSR modifier that will cause the total payout at the end the performance period to increase or decrease depending on Ambac's stock performance relative to a peer group. Compensation costs for all performance stock units are only recognized when the achievement of the performance conditions are considered probable. Once deemed probable, such compensation costs are recognized as compensation expense over the relevant service period. Compensation costs are initially based on the probable outcome of the performance conditions and adjusted for subsequent changes in the estimated or actual outcome each reporting period as necessary. Changes in the estimated or actual outcome of a performance condition are recognized by reflecting a retrospective adjustment to compensation cost in the current period. In 2015, Ambac UK's Board of Directors adopted a long term incentive plan which provided cash based performance awards to Ambac UK employees. Since all performance conditions under this plan were met, the Ambac UK Board of Directors adopted a new long term incentive plan for Ambac UK employees in 2020, which includes both performance and time based awards. Compensation costs for all performance based awards are based on the probable outcome of the performance conditions and adjusted for subsequent changes in the estimated or actual outcome each reporting period as necessary. Compensation costs for time-based awards are recognized evenly over the service period.
Lessee, Operating Lease, Disclosure [Table Text Block]	Operating Leases In 2019, Ambac adopted ASU 2016-02, Leases (Topic 842), amended by ASU 2018-01, Land Easement Practical Expedient; ASU 2018-10, Codification Improvements to Topic 842; ASU 2018-11, Targeted Improvements; ASU 2018-20, Narrow-Scope Improvements for Lessors; and ASU 2019-01, Leases (Topic 842): Codification Improvements (collectively the "New Lease Standard"). Ambac used a modified retrospective approach and applied the New Lease Standard on its effective date of January 1, 2019. Additionally, Ambac applied the New Lease Standard to its recently acquired affiliate, Xchange, on the acquisition date of December 31, 2020. Refer to Note 3. Business Combination for further discussion of the acquisition. A contract contains a lease if it conveys the right to control the use of identified property, plant, or equipment for a period of time in exchange for consideration. Ambac's evaluation of whether certain contracts contain leases requires judgment regarding what party controls the asset and whether the asset is physically distinct. Ambac is the lessee in leases which are classified as operating leases. In accordance with the New Lease Standard, Ambac recognizes a single lease cost, calculated so that the cost is allocated generally on a straight-line basis over the lease term within operating expenses in the Consolidated Statements of Total Comprehensive Income (Loss). The lease term commences on the earlier of the date when we become legally obligated for the rent payments or the date on which we take possession of the property. For such operating leases, Ambac recognizes a right-of-use ("ROU") asset and a lease liability, initially measured at the present value of the lease payments, on the later of the adoption date or lease commencement date. The discount rate used to initially measure the ROU assets and lease liabilities reflects the estimated secured borrowing rate of the applicable Ambac subsidiary, which considers the rate of existing or recent debt obligations of the entity. All cash payments are classified within operating activities in the statement of cash flows. For contracts where Ambac is the lessee, we have elected the short-term lease recognition exemption for all leases that qualify. For those leases that qualify for that exemption, we will not recognize ROU assets or lease liabilities. For all contracts where Ambac is the lessee and lessor we have also elected the practical expedient to not separate lease and non-lease components.
Depreciation and Amortization	Depreciation and AmortizationDepreciation of furniture and fixtures, certain information technology development costs and electronic data processing equipment is charged over the estimated useful lives of the respective assets, ranging from three to five years, using the straight-line method. Amortization of leasehold improvements is charged over the remaining term of the respective operating lease using the straight-line method.
Foreign Currency	Foreign Currency Financial statement accounts expressed in foreign currencies are translated into U.S. dollars in accordance with the Foreign Currency Matters Topic of the ASC. The functional currencies of Ambac's subsidiaries are the local currencies of the country where the respective subsidiaries are based, which are also the primary operating environments in which the subsidiaries operate. Foreign currency translation: Functional currency assets and liabilities of Ambac’s foreign subsidiaries are translated into U.S. dollars using exchange rates in effect at the balance sheet dates and the related translation adjustments, net of deferred taxes, are included as a component of Accumulated Other Comprehensive Income (Loss) in Stockholders' Equity. Functional currency operating results of foreign subsidiaries are translated using average exchange rates. Foreign currency transactions: The impact of non-functional currency transactions and the remeasurement of non-functional currency assets and liabilities into the respective subsidiaries' functional currency (collectively "foreign currency transactions gains/(losses)") are $(1), $12 and $(7) for the years ended December 31, 2020, 2019 and 2018. Foreign currency transaction gains/(losses) are primarily the result of remeasuring Ambac UK's assets and liabilities denominated in currencies other than its functional currency, primarily the U.S. dollar and the Euro.
Income Taxes	Income Taxes Ambac files a consolidated U.S. Federal income tax return with its subsidiaries. Ambac UK files tax returns in both the United Kingdom and Italy (for its Milan branch). Current tax assets and liabilities are recognized for taxes refundable or payable for the current year. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on current and deferred tax assets and liabilities of a change in tax rates is recognized in the period that includes the enactment date. In July 2020, United Kingdom legislation increasing the tax rate from 17% to 19% was fully enacted. As such, we incorporated the effects of the tax rate increase in our current and deferred tax evaluation for the year ended December 31, 2020. The Income Taxes Topic of the ASC requires that companies assess whether valuation allowances should be established against their deferred tax assets based on the consideration of all available evidence using a ‘more likely than not” standard. In making such judgments, significant weight is given to evidence that can be objectively verified. The level of deferred tax asset recognition is influenced by management’s assessment of future profitability, which depends on the existence of sufficient taxable income within the carry forward periods available under the tax law.
Net Income Per Share	Net Income Per ShareBasic net income per share is computed by dividing net income attributable to common stockholders by the weighted-average number of common shares outstanding and vested restricted stock units (together, "Basic Weighted Average Shares Outstanding"). Diluted net income per share is computed by dividing net income attributable to common stockholders by the Basic Weighted-Average Shares Outstanding plus all potential dilutive common shares outstanding during the period. All potential dilutive common shares outstanding consider common stock deliverable pursuant to warrants, vested and unvested options, unvested restricted stock units and performance stock units granted under existing compensation plans.
Reclassifications	Reclassifications and Rounding Reclassifications may have been made to prior years' amounts to conform to the current year's presentation. Certain amounts and tables in the consolidated financial statements and associated notes may not add due to rounding.
Adoption of New Accounting Standards	Adopted Accounting Standards Effective January 1, 2020, Ambac adopted the following accounting standards: CECL For further discussion of CECL, refer to the Measurement of Credit Losses (CECL), Investments, Net Premiums, Loans, and Loss and Loss Expenses sections in Note 2. Basis of Presentation and Significant Accounting Policies; Note 8. Financial Guarantee Insurance Contracts; and Note 11. Investments in the Notes to Consolidated Financial Statements included in this Annual Report on Form 10-K for the year ended December 31, 2020. For available-for-sale debt securities, the updated guidance was applied prospectively and for financial instruments measured at amortized cost (i.e. premiums receivable, loans and reinsurance recoverables), the updated guidance was applied by a cumulative effect adjustment to the opening balance of retained earnings at January 1, 2020. This adjustment was not material to retained earnings or any individual balance sheet line item. Fair Value Measurement Disclosures In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820) - Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement. The ASU modified various disclosure requirements on fair value measurements. Relevant disclosures that were removed, modified and added are as follows: •Removals: (1) Amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, (2) Policy for timing of transfers between levels, and (3) Valuation processes for Level 3 fair value measurements. •Modifications: (1) For investments in certain entities that calculate net asset value, disclosures are required for the timing of liquidation of an investee's assets and the date when restrictions from redemption might lapse, only if the investee has communicated the timing to the reporting entity or publicly announced it and (2) Clarification that the measurement uncertainty disclosure is to communicate information about the uncertainty in measurement as of the reporting date and not possible future changes. •Additions: (1) Changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period and (2) Range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. Alternatively, an entity may disclose other quantitative information (such as the median or arithmetic average) if it determines that it is a more reasonable and rational method to reflect the distribution of unobservable inputs used. Disclosure amendments related to changes in unrealized gains and losses included in other comprehensive income (loss) for Level 3 instruments, the range and weighted average of significant unobservable inputs, and the narrative description of measurement uncertainty were applied prospectively only for the most recent interim or annual period presented. All other disclosure amendments were applied retrospectively to all periods presented. Refer to Note 10. Fair Value Measurements for further disclosures. VIE Related Party Guidance In October 2018, the FASB issued ASU 2018-17, Consolidation (Topic 810) - Targeted Improvements to Related Party Guidance for Variable Interest Entities. To determine whether a decision-making fee is a variable interest, under the new guidance a reporting entity must consider indirect interests held through related parties under common control on a proportional basis rather than as a direct interest in its entirety (as was previously required under prior GAAP). These amendments create alignment between determining whether a decision making fee is a variable interest and determining whether a reporting entity within a related party group is the primary beneficiary of a VIE. Adoption of this ASU did not impact Ambac's financial statements. Cloud Computing Arrangement Service Contracts In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other— Internal-Use Software (Subtopic 350-40) - Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. The new guidance requires a customer in a cloud computing arrangement that is a service contract to capitalize certain implementation costs as if the arrangement was an internal-use software project. The internal-use software guidance requires the capitalization of certain costs incurred only during the application development stage. That guidance also requires entities to expense costs during the preliminary project and post-implementation stages as they are incurred. Adoption of this ASU did not impact Ambac's financial statements. Effective December 31, 2020, Ambac adopted the following accounting standard: Defined Benefit and Other Postretirement Plans Disclosures In August 2018, the FASB issued ASU 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans - General (Subtopic 715-20) - Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans. The ASU modifies various disclosure requirements for employers that sponsor defined benefit pension or other postretirement plans. Relevant disclosures that have been removed are the effects of a one percentage point change in assumed health care cost trend rates on the (a) aggregate of the service and interest cost components of the net periodic pension cost and (b) benefit obligation for postretirement healthcare benefits. Adoption of this ASU only affected disclosures and did not have an impact on Ambac's financial statements. Future Application of Accounting Standards: Reference Rate Reform In March 2020, the FASB issued ASU 2020-04, Reference Rate Reform (Topic 848) - Facilitation of the Effects of Reference Rate Reform on Financial Reporting. The ASU provides companies with optional guidance to ease the potential accounting burden related to transitioning away from reference rates, such as LIBOR, that are expected to be discontinued as a result of initiatives undertaken by various jurisdictions around the world. For example, under current GAAP, contract modifications which change a reference rate are required to be evaluated in determining whether the modifications result in the establishment of new contracts or the continuation of existing contracts. The amendments in this ASU provide optional expedients and exceptions for applying GAAP to contracts, hedging relationships, and other transactions affected by reference rate reform if certain criteria are met. The ASU can be applied prospectively as of the beginning of the interim period that includes March 12, 2020, (January 1, 2020 for calendar year companies) or any date thereafter, but does not apply to contract modifications and other transactions entered into or evaluated after December 31, 2022. Management has not determined when it will adopt this ASU, and the impact on Ambac's financial statements is being evaluated.