Basis of Presentation and Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2021
Accounting Policies [Abstract]
Organization, Consolidation and Presentation of Financial Statements Disclosure	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. See Note 3. 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 and deconsolidated.
Basis of Presentation and Significant Accounting Policies [Text Block]	Basis of Presentation: The accompanying unaudited consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles ("GAAP") for interim financial reporting and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and disclosures required by GAAP for annual periods. These consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2020. The accompanying consolidated financial statements have not been audited by an independent registered public accounting firm in accordance with the standards of the Public Company Accounting Oversight Board (U.S.), but in the opinion of management such financial statements include all adjustments necessary for the fair presentation of the Company’s consolidated financial position and results of operations. The results of operations for the three months ended March 31, 2021, may not be indicative of the results that may be expected for the year ending December 31, 2021. The December 31, 2020, consolidated balance sheet was derived from audited financial statements. The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. As additional information becomes available or actual amounts become determinable, the recorded estimates are revised and reflected in operating results.
Foreign Currency Transactions and Translations Policy [Policy Text Block]	Foreign Currency: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 $(5) and $1 for the three months ended March 31, 2021 and 2020, respectively. Foreign currency transactions 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.
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.
New Accounting Pronouncements and Changes in Accounting Principles	Adopted Accounting Standards: Effective January 1, 2021, the Company adopted the following accounting standard:
Description of New Accounting Pronouncements Not yet Adopted [Text Block]	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 or is subsequent to March 12, 2020, 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.
Insurance Premiums Revenue Recognition, Policy	Revenue Recognition: Revenues for the MGA/U business operations are recognized in accordance with the Revenue from Contracts with Customers Topic of the ASC. The following steps are applied to recognize revenue: identify the contract(s) with the customer, identify the performance obligations in the contract(s), determine the transaction price, allocate the transaction price to the performance obligations in the contract and recognize revenue when (or as) the entity satisfies a performance obligation. A performance obligation is satisfied either at a point in time or over time depending on the nature of the product or service provided, and the specific terms of the contract with customers. MGA/U performance obligations consist of placing policies with insurers and, for certain products, providing claims servicing. Revenue from limited and short-term medical policies sold through affinity groups ("Affinity") are recognized up front as no further performance obligations exist after policy placement. Revenue from employer stop loss policies ("ESL") is apportioned to policy placement, recognized upfront, and claims servicing, recognized over the claim period, based on the relative stand-alone selling price of the respective performance obligations. Revenue consists of base and profit-sharing commissions. Base commissions, associated with policy placement and claims servicing, are estimated by applying the contractual commission percentages to estimated gross premiums written. Profit-sharing commissions represent variable consideration associated with policy placement only and are estimated based on expected loss ratios and the estimated gross premium for base commissions. Base and profit-sharing commissions are estimated with a constraint applied such that a significant reversal of revenue in the future is not probable. MGA/U revenue is reported in other income (expense) on the Consolidated Statement of Total Comprehensive Income. Contract assets represent the Company's right to future consideration for services it has already transferred to the customer, which is also conditional on future performance. Once the right to consideration becomes unconditional, it is reported as a receivable. Contract liabilities represent the Company's obligation to transfer services for which it has already received consideration from the customer. Contract assets and contract liabilities are reported as other assets and other liabilities, respectively, on the Consolidated Balance Sheet.

Basis of Presentation and Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2021

Organization, Consolidation and Presentation of Financial Statements Disclosure

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. See Note 3. 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 and deconsolidated.

Basis of Presentation and Significant Accounting Policies [Text Block]

Basis of Presentation:

The accompanying unaudited consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles ("GAAP") for interim financial reporting and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and disclosures required by GAAP for annual periods. These consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2020. The accompanying consolidated financial statements have not been audited by an independent registered public accounting firm in accordance with the standards of the Public Company Accounting Oversight Board (U.S.), but in the opinion of management such financial statements include all adjustments necessary for the fair presentation of the Company’s consolidated financial position and results of operations. The results of operations for the three months ended March 31, 2021, may not be indicative of the results that may be expected for the year ending December 31, 2021. The December 31, 2020, consolidated balance sheet was derived from audited financial statements.

The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. As additional information becomes available or actual amounts become determinable, the recorded estimates are revised and reflected in operating results.

Foreign Currency Transactions and Translations Policy [Policy Text Block]

Foreign Currency: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 $(5) and $1 for the three months ended March 31, 2021 and 2020, respectively. Foreign currency transactions 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.

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.

New Accounting Pronouncements and Changes in Accounting Principles

Adopted Accounting Standards:

Effective January 1, 2021, the Company adopted the following accounting standard:

Description of New Accounting Pronouncements Not yet Adopted [Text Block]

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 or is subsequent to March 12, 2020, 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.

Insurance Premiums Revenue Recognition, Policy

Revenue Recognition:

Revenues for the MGA/U business operations are recognized in accordance with the Revenue from Contracts with Customers Topic of the ASC. The following steps are applied to recognize revenue: identify the contract(s) with the customer, identify the performance obligations in the contract(s), determine the transaction price, allocate the transaction price to the performance obligations in the contract and recognize revenue when (or as) the entity satisfies a performance obligation. A performance obligation is satisfied either at a point in time or over time depending on the nature of the product or service provided, and the specific terms of the contract with customers.

MGA/U performance obligations consist of placing policies with insurers and, for certain products, providing claims servicing. Revenue from limited and short-term medical policies sold through affinity groups ("Affinity") are recognized up front as no further performance obligations exist after policy placement. Revenue from employer stop loss policies ("ESL") is apportioned to policy placement, recognized upfront, and claims servicing, recognized over the claim period, based on the relative stand-alone selling price of the respective performance obligations.

Revenue consists of base and profit-sharing commissions. Base commissions, associated with policy placement and claims servicing, are estimated by applying the contractual commission percentages to estimated gross premiums written. Profit-sharing commissions represent variable consideration associated with policy placement only and are estimated based on expected loss ratios and the estimated gross premium for base commissions. Base and profit-sharing commissions are estimated with a constraint applied such that a significant reversal of revenue in the future is not probable. MGA/U revenue is reported in other income (expense) on the Consolidated Statement of Total Comprehensive Income.

Contract assets represent the Company's right to future consideration for services it has already transferred to the customer, which is also conditional on future performance. Once the right to consideration becomes unconditional, it is reported as a receivable. Contract liabilities represent the Company's obligation to transfer services for which it has already received consideration from the customer. Contract assets and contract liabilities are reported as other assets and other liabilities, respectively, on the Consolidated Balance Sheet.