Business and Basis of Presentation (Policies)	6 Months Ended
Jun. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The unaudited interim condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). In management's opinion, all material adjustments necessary for a fair statement of the financial condition, results of operations and cash flows of the Company, including its consolidated variable interest entities (VIEs), are reflected in the periods presented and are of a normal, recurring nature. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of 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. These unaudited interim condensed consolidated financial statements are as of June 30, 2021 and cover the three-month period ended June 30, 2021 (Second Quarter 2021), the three-month period ended June 30, 2020 (Second Quarter 2020), the six-month period ended June 30, 2021 (Six Months 2021) and the six-month period ended June 30, 2020 (Six Months 2020). Certain financial information that is normally included in annual financial statements prepared in accordance with GAAP, but is not required for interim reporting purposes, has been condensed or omitted. The year-end condensed consolidated balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. Certain prior year balances have been reclassified to conform to the current year's presentation.
Consolidation Policy	The unaudited interim condensed consolidated financial statements include the accounts of AGL, its direct and indirect subsidiaries, and its consolidated financial guaranty VIEs (FG VIEs) and consolidated investment vehicles (CIVs). Intercompany accounts and transactions between and among all consolidated entities have been eliminated.
Recent Accounting Standards Adopted	Recent Accounting Standards Adopted Simplification of the Accounting for Income Taxes In December 2019, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. The amendments in this ASU simplify the accounting for income taxes by removing certain exceptions and clarifying certain requirements regarding franchise taxes, goodwill, consolidated tax expenses and annual effective tax rate calculations. The ASU was effective for interim and annual periods beginning after December 15, 2020. This ASU did not have an impact on the Company's consolidated financial statements. Recent Accounting Standards Not Yet Adopted Targeted Improvements to the Accounting for Long-Duration Contracts In August 2018, the FASB issued ASU 2018-12, Financial Services - Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts. The amendments in this ASU: •improve the timeliness of recognizing changes in the liability for future policy benefits and modify the rate used to discount future cash flows, •simplify and improve the accounting for certain market-based options or guarantees associated with deposit (or account balance) contracts, •simplify the amortization of deferred acquisition costs (DAC), and •improve the effectiveness of the required disclosures. This ASU does not affect the Company’s financial guaranty insurance contracts, but may affect its accounting for certain specialty (non-financial guaranty) insurance contracts. In November 2020, the FASB deferred the effective date of this ASU to January 1, 2023 with early adoption permitted. If early adoption is elected, there is transition relief allowing for the transition date to be either the beginning of the prior period presented or the beginning of the earliest period presented. If early adoption is not elected, the transition date is required to be the beginning of the earliest period presented. The Company is evaluating when it will adopt this ASU and does not expect this ASU to have a material effect on its consolidated financial statements. 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. This ASU provides 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 amendments in this ASU only apply to contracts that reference the London Interbank Offered Rate (LIBOR) or another reference rate that is expected to be discontinued due to reference rate reform. This ASU is effective upon issuance and may be applied prospectively for contract modifications that occur from March 12, 2020 through December 31, 2022. In January 2021, the FASB issued ASU 2021-01, Reference Rate Reform (Topic 848): Scope, which clarifies the scope of relief related to ASU 2020-04. This ASU became effective upon issuance and may be applied on a full retrospective basis as of any date from the beginning of an interim period that includes or is subsequent to March 12, 2020 or prospectively for contract modifications made on or before December 31, 2022. The Company has not yet applied the relief afforded by these standard amendments and is evaluating the effect that these ASUs will have on its consolidated financial statements.
Fair Value Measurement	The Company carries a significant portion of its assets and liabilities at fair value. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date (i.e., exit price). The price represents the price available in the principal market for the asset or liability. If there is no principal market, then the price is based on a hypothetical market that maximizes the value received for an asset or minimizes the amount paid for a liability (i.e., the most advantageous market). Fair value is based on quoted market prices, where available. If listed prices or quotes are not available, fair value is based on either internally developed models that primarily use, as inputs, market-based or independently sourced market parameters, including but not limited to yield curves, interest rates and debt prices or with the assistance of an independent third party using a discounted cash flow approach and the third party’s proprietary pricing models. In addition to market information, models also incorporate transaction details, such as maturity of the instrument and contractual features designed to reduce the Company’s credit exposure, such as collateral rights as applicable. Valuation adjustments may be made to ensure that financial instruments are recorded at fair value. These adjustments include amounts to reflect counterparty credit quality, the Company’s creditworthiness and constraints on liquidity. As markets and products develop and the pricing for certain products becomes more or less transparent, the Company may refine its methodologies and assumptions. During Six Months 2021, no changes were made to the Company’s valuation models that had, or are expected to have, a material impact on the Company’s condensed consolidated balance sheets or statements of operations and comprehensive income. The Company’s methods for calculating fair value produce a fair value that may not be indicative of net realizable value or reflective of future fair values. The use of different methodologies or assumptions to determine fair value of certain financial instruments could result in a materially different estimate of fair value at the reporting date. The categorization within the fair value hierarchy is determined based on whether the inputs to valuation techniques used to measure fair value are observable or unobservable. Observable inputs reflect market data obtained from independent sources, while unobservable inputs reflect Company estimates of market assumptions. The fair value hierarchy prioritizes model inputs into three broad levels as follows, with Level 1 being the highest and Level 3 the lowest. An asset's or liability’s categorization is based on the lowest level of significant input to its valuation. Level 1—Quoted prices for identical instruments in active markets. The Company generally defines an active market as a market in which trading occurs at significant volumes. Active markets generally are more liquid and have a lower bid-ask spread than an inactive market. Level 2—Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and observable inputs other than quoted prices, such as interest rates or yield curves and other inputs derived from or corroborated by observable market inputs. Level 3—Model derived valuations in which one or more significant inputs or significant value drivers are unobservable. Financial instruments are considered Level 3 when their values are determined using pricing models, discounted cash flow methodologies or similar techniques and at least one significant model assumption or input is unobservable. Level 3 financial instruments also include those for which the determination of fair value requires significant management judgment or estimation.

Business and Basis of Presentation (Policies)

6 Months Ended

Jun. 30, 2021

Organization, Consolidation and Presentation of Financial Statements [Abstract]

The unaudited interim condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). In management's opinion, all material adjustments necessary for a fair statement of the financial condition, results of operations and cash flows of the Company, including its consolidated variable interest entities (VIEs), are reflected in the periods presented and are of a normal, recurring nature. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of 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.

These unaudited interim condensed consolidated financial statements are as of June 30, 2021 and cover the three-month period ended June 30, 2021 (Second Quarter 2021), the three-month period ended June 30, 2020 (Second Quarter 2020), the six-month period ended June 30, 2021 (Six Months 2021) and the six-month period ended June 30, 2020

(Six Months 2020). Certain financial information that is normally included in annual financial statements prepared in accordance with GAAP, but is not required for interim reporting purposes, has been condensed or omitted. The year-end condensed consolidated balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. Certain prior year balances have been reclassified to conform to the current year's presentation.

Consolidation Policy

The unaudited interim condensed consolidated financial statements include the accounts of AGL, its direct and indirect subsidiaries, and its consolidated financial guaranty VIEs (FG VIEs) and consolidated investment vehicles (CIVs). Intercompany accounts and transactions between and among all consolidated entities have been eliminated.

Recent Accounting Standards Adopted

Simplification of the Accounting for Income Taxes

In December 2019, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. The amendments in this ASU simplify the accounting for income taxes by removing certain exceptions and clarifying certain requirements regarding franchise taxes, goodwill, consolidated tax expenses and annual effective tax rate calculations. The ASU was effective for interim and annual periods beginning after December 15, 2020. This ASU did not have an impact on the Company's consolidated financial statements.

Recent Accounting Standards Not Yet Adopted

Targeted Improvements to the Accounting for Long-Duration Contracts

In August 2018, the FASB issued ASU 2018-12, Financial Services - Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts. The amendments in this ASU:

•improve the timeliness of recognizing changes in the liability for future policy benefits and modify the rate used to discount future cash flows,

•simplify and improve the accounting for certain market-based options or guarantees associated with deposit (or account balance) contracts,

•simplify the amortization of deferred acquisition costs (DAC), and

•improve the effectiveness of the required disclosures.

This ASU does not affect the Company’s financial guaranty insurance contracts, but may affect its accounting for certain specialty (non-financial guaranty) insurance contracts. In November 2020, the FASB deferred the effective date of this ASU to January 1, 2023 with early adoption permitted. If early adoption is elected, there is transition relief allowing for the transition date to be either the beginning of the prior period presented or the beginning of the earliest period presented. If early adoption is not elected, the transition date is required to be the beginning of the earliest period presented. The Company is evaluating when it will adopt this ASU and does not expect this ASU to have a material effect on its consolidated financial statements.

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. This ASU provides 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 amendments in this ASU only apply to contracts that reference the London Interbank Offered Rate (LIBOR) or another reference rate that is expected to be discontinued due to reference rate reform. This ASU is effective upon issuance and may be applied prospectively for contract modifications that occur from March 12, 2020 through December 31, 2022.

In January 2021, the FASB issued ASU 2021-01, Reference Rate Reform (Topic 848): Scope, which clarifies the scope of relief related to ASU 2020-04. This ASU became effective upon issuance and may be applied on a full retrospective basis as of any date from the beginning of an interim period that includes or is subsequent to March 12, 2020 or prospectively for contract modifications made on or before December 31, 2022.

The Company has not yet applied the relief afforded by these standard amendments and is evaluating the effect that these ASUs will have on its consolidated financial statements.

Fair Value Measurement

The Company carries a significant portion of its assets and liabilities at fair value. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date (i.e., exit price). The price represents the price available in the principal market for the asset or liability. If there is no principal market, then the price is based on a hypothetical market that maximizes the value received for an asset or minimizes the amount paid for a liability (i.e., the most advantageous market).

Fair value is based on quoted market prices, where available. If listed prices or quotes are not available, fair value is based on either internally developed models that primarily use, as inputs, market-based or independently sourced market parameters, including but not limited to yield curves, interest rates and debt prices or with the assistance of an independent third party using a discounted cash flow approach and the third party’s proprietary pricing models. In addition to market information, models also incorporate transaction details, such as maturity of the instrument and contractual features designed to reduce the Company’s credit exposure, such as collateral rights as applicable.

Valuation adjustments may be made to ensure that financial instruments are recorded at fair value. These adjustments include amounts to reflect counterparty credit quality, the Company’s creditworthiness and constraints on liquidity. As markets and products develop and the pricing for certain products becomes more or less transparent, the Company may refine its methodologies and assumptions. During Six Months 2021, no changes were made to the Company’s valuation models that had, or are expected to have, a material impact on the Company’s condensed consolidated balance sheets or statements of operations and comprehensive income.

The Company’s methods for calculating fair value produce a fair value that may not be indicative of net realizable value or reflective of future fair values. The use of different methodologies or assumptions to determine fair value of certain financial instruments could result in a materially different estimate of fair value at the reporting date.

The categorization within the fair value hierarchy is determined based on whether the inputs to valuation techniques used to measure fair value are observable or unobservable. Observable inputs reflect market data obtained from independent sources, while unobservable inputs reflect Company estimates of market assumptions. The fair value hierarchy prioritizes model inputs into three broad levels as follows, with Level 1 being the highest and Level 3 the lowest. An asset's or liability’s categorization is based on the lowest level of significant input to its valuation.

Level 1—Quoted prices for identical instruments in active markets. The Company generally defines an active market as a market in which trading occurs at significant volumes. Active markets generally are more liquid and have a lower bid-ask spread than an inactive market.

Level 2—Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and observable inputs other than quoted prices, such as interest rates or yield curves and other inputs derived from or corroborated by observable market inputs.

Level 3—Model derived valuations in which one or more significant inputs or significant value drivers are unobservable. Financial instruments are considered Level 3 when their values are determined using pricing models, discounted cash flow methodologies or similar techniques and at least one significant model assumption or input is unobservable. Level 3 financial instruments also include those for which the determination of fair value requires significant management judgment or estimation.