Business and Basis of Presentation (Policies)	9 Months Ended
Business and Basis of Presentation (Policies)	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The unaudited interim consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP) and, in the opinion of management, reflect all adjustments that are of a normal recurring nature, necessary for a fair statement of the financial condition, results of operations and cash flows of the Company and its consolidated variable interest entities (VIEs) for the periods presented. 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 consolidated financial statements are as of September 30, 2017 and cover the three-month period ended September 30, 2017 (Third Quarter 2017), the three-month period ended September 30, 2016 (Third Quarter 2016), the nine-month period ended September 30, 2017 (Nine Months 2017) and the nine-month period ended September 30, 2016 (Nine Months 2016). 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.
Consolidation	The unaudited interim consolidated financial statements include the accounts of AGL, its direct and indirect subsidiaries, and its consolidated VIEs. Intercompany accounts and transactions between and among all consolidated entities have been eliminated.
New Accounting Pronouncements Adopted and Future Application	Adopted Accounting Standards Statement of Cash Flows In November 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the Emerging Issues Task Force), which addresses the presentation of changes in restricted cash and restricted cash equivalents in the statement of cash flows with the objective of reducing the existing diversity in practice. Under the ASU, entities are required to show the changes in the total of cash, cash equivalents, restricted cash and restricted cash equivalents in the statement of cash flows. As a result, entities will no longer present transfers between cash and cash equivalents and restricted cash and restricted cash equivalents in the statement of cash flows. When cash, cash equivalents, restricted cash and restricted cash equivalents are presented in more than one line item on the balance sheet, the ASU requires a reconciliation be presented either on the face of the statement of cash flows or in the notes to the financial statements showing the totals in the statement of cash flows to the related captions in the balance sheet. The ASU was adopted on January 1, 2017 and was applied retrospectively. The required reconciliation is shown in Note 10, Investments and Cash. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (a consensus of the Emerging Issues Task Force), which addresses eight specific cash flow issues with the objective of reducing the existing diversity in practice. The ASU was adopted on January 1, 2017 and did not have an effect on the Company’s consolidated statements of cash flows for the periods presented. Share-Based Payments In March 2016, the FASB issued ASU 2016-09, Compensation - Stock Compensation (Topic 718) - Improvements to Employee Share-Based Payment, which simplifies several aspects of the accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures, and statutory tax withholding requirements, as well as classification in the statement of cash flows. The new guidance requires all income tax effects of awards to be recognized in the income statement when the awards vest or are settled. It also allows an employer to repurchase more of an employee’s shares than it previously could for tax withholding purposes without triggering liability accounting and to make a policy election to account for forfeitures as they occur. The ASU was adopted January 1, 2017 with no material effect on the consolidated financial statements. Future Application of Accounting Standards Income Taxes In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740) - Intra-Entity Transfers of Assets Other Than Inventory, which removes the current prohibition against immediate recognition of the current and deferred income tax effects of intra-entity transfers of assets other than inventory. Under the ASU, the selling (transferring) entity is required to recognize a current income tax expense or benefit upon transfer of the asset. Similarly, the purchasing (receiving) entity is required to recognize a deferred tax asset or deferred tax liability, as well as the related deferred tax benefit or expense, upon receipt of the asset. The ASU is effective for annual periods beginning after December 15, 2017, including interim periods within those annual periods, and early adoption is permitted. The ASU’s amendments are to be applied on a modified retrospective basis recognizing the effects in retained earnings as of the beginning of the year of adoption. The Company does not expect this ASU to have a material effect on its consolidated financial statements. Financial Instruments In January 2016, the FASB issued ASU 2016-01, Financial Instruments - Overall (Subtopic 825-10) - Recognition and Measurement of Financial Assets and Financial Liabilities. The amendments in this ASU are intended to make targeted improvements to GAAP by addressing certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. Under the ASU, certain equity securities will need to be accounted for at fair value with changes in fair value recognized through net income instead of other comprehensive income (OCI). The Company does not expect that the amendment related to certain equity securities will have a material effect on its consolidated financial statements. Another amendment pertains to liabilities that an entity has elected to measure at fair value in accordance with the fair value option for financial instruments. For these liabilities, the portion of fair value change related to instrument specific credit risk will be separately presented in OCI as opposed to the income statement. The Company elected the fair value option to account for its consolidated FG VIEs. The Company is evaluating the effect that the ASU will have on its consolidated FG VIEs. The ASU is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Entities will be required to record a cumulative-effect adjustment to the statement of financial position as of the beginning of the fiscal year in which the guidance is adopted. Premium Amortization on Purchased Callable Debt Securities In March 2017, the FASB issued ASU 2017-08, Receivables-Nonrefundable Fees and Other Costs (Topic 310-20) - Premium Amortization on Purchased Callable Debt Securities. This ASU shortens the amortization period for the premium on certain purchased callable debt securities to the earliest call date. This ASU has no effect on the accounting for purchased callable debt securities held at a discount. ASU 2017-08 is to be applied using a modified retrospective approach through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. The ASU is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The Company does not expect this ASU to have a material effect on its consolidated financial statements. Leases In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). This ASU requires lessees to present right-of-use assets and lease liabilities on the balance sheet. ASU 2016-02 is to be applied using a modified retrospective approach at the beginning of the earliest comparative period in the financial statements. The ASU is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The Company is evaluating the effect that this ASU will have on its consolidated financial statements. Credit Losses on Financial Instruments In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The amendments in this ASU are intended to improve financial reporting by requiring timelier recording of credit losses on loans and other financial instruments held by financial institutions and other organizations. The ASU requires the measurement of all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. Financial institutions will use forward-looking information to better inform their credit loss estimates as a result of the ASU. While many of the loss estimation techniques applied today will still be permitted, the inputs to those techniques will change to reflect the full amount of expected credit losses. The ASU requires enhanced disclosures to help investors and other financial statement users to better understand significant estimates and judgments used in estimating credit losses, as well as credit quality and underwriting standards of an organization’s portfolio. In addition, the ASU amends the accounting for credit losses on available-for-sale securities and purchased financial assets with credit deterioration. The ASU also eliminates the concept of “other than temporary” from the impairment model for certain available-for-sale securities. Accordingly, the ASU states that an entity must use an allowance approach, must limit the allowance to an amount by which the security’s fair value is less than its amortized cost basis, may not consider the length of time fair value has been less than amortized cost, and may not consider recoveries in fair value after the balance sheet date when assessing whether a credit loss exists. For purchased financial assets with credit deterioration, the ASU requires an entity’s method for measuring credit losses to be consistent with its method for measuring expected losses for originated and purchased non-credit-deteriorated assets. The ASU is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. For debt securities classified as available-for-sale, entities will be required to record a cumulative-effect adjustment to the statement of financial position as of the beginning of the first reporting period in which the guidance is adopted. The changes to the impairment model for available-for-sale securities and changes to purchased financial assets with credit deterioration are to be applied prospectively. The Company is evaluating the effect that this ASU will have on its consolidated financial statements.
Fair Value of Financial Instruments	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 Nine Months 2017, no changes were made to the Company’s valuation models that had or are expected to have, a material impact on the Company’s 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 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. Transfers between Levels 1, 2 and 3 are recognized at the end of the period when the transfer occurs. The Company reviews the classification between Levels 1, 2 and 3 quarterly to determine whether a transfer is necessary.
Consolidation, Variable Interest Entity	As part of the terms of its financial guaranty contracts, the Company obtains certain protective rights with respect to the VIE that are triggered by the occurrence of certain events, such as failure to be in compliance with a covenant due to poor deal performance or a deterioration in a servicer or collateral manager's financial condition. At deal inception, the Company typically is not deemed to control a VIE; however, once a trigger event occurs, the Company's control of the VIE typically increases. The Company continuously evaluates its power to direct the activities that most significantly impact the economic performance of VIEs that have debt obligations insured by the Company and, accordingly, where the Company is obligated to absorb VIE losses or receive benefits that could potentially be significant to the VIE. The Company obtains protective rights under its insurance contracts that give the Company additional controls over a VIE if there is either deterioration of deal performance or in the financial health of the deal servicer. The Company is deemed to be the control party for certain VIEs under GAAP, typically when its protective rights give it the power to both terminate and replace the deal servicer, which are characteristics specific to the Company's financial guaranty contracts. If the protective rights that could make the Company the control party have not been triggered, then the VIE is not consolidated. If the Company is deemed no longer to have those protective rights, the transaction is deconsolidated.