Business and Basis of Presentation (Policies)	3 Months Ended
Business and Basis of Presentation (Policies)	Mar. 31, 2018
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) and, in the opinion of management, reflect all material 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 condensed consolidated financial statements are as of March 31, 2018 and cover the three-month period ended March 31, 2018 (First Quarter 2018) and the three-month period ended March 31, 2017 (First Quarter 2017). 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 balance sheet data was derived from audited financial statements.
Consolidation	The unaudited interim condensed 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. Certain prior year balances have been reclassified to conform to the current year's presentation.
New Accounting Pronouncements Adopted and Future Application	Adopted Accounting Standards Financial Instruments In January 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (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. Amendments under this ASU apply to the Company's FG VIE liabilities, which the Company has historically elected to measure through the statement of operations under the fair value option, and to certain equity securities in the Company’s investment portfolio. For FG VIE liabilities with recourse, the portion of the change in fair value caused by changes in instrument specific credit risk (ISCR) must now be separately presented in other comprehensive income (OCI) as opposed to the statement of operations. See Note 9, Consolidated Variable Interest Entities for additional information. Amendments under this ASU also apply to equity securities, except those that are accounted for under the equity method of accounting or that resulted in consolidation of the investee by the Company. For equity securities accounted for at fair value, changes in fair value that previously were recorded in OCI, are recorded in other income in the condensed consolidated statements of operations effective January 1, 2018. Equity securities carried at cost as of December 31, 2017, are recorded at cost less impairment plus or minus the change resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer. See Note 10, Investments and Cash for additional information. Effective January 1, 2018, the Company adopted this ASU with a cumulative-effect adjustment to the statement of financial position as of January 1, 2018. This resulted in a reclassification of a net $32 million loss, net of tax, from retained earnings to accumulated OCI (AOCI). See Note 17, Shareholders' Equity, for additional information. 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 to be applied on a modified retrospective basis (i.e. by recording 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 ASU was adopted on January 1, 2018 with no material effect on the condensed consolidated financial statements. Future Application of Accounting Standards 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. It is to be applied using a modified retrospective approach and the ASU is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2018. The Company does not expect this ASU to have a material effect on its condensed 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 and is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company intends to adopt this ASU on January 1, 2019. The Company is evaluating the effect that this ASU will have on its condensed consolidated financial statements. The Company currently accounts for its lease agreements where the Company is the lessee as operating leases and, therefore, recognizes its lease expense on a straight-line basis. See Note 14, Commitments and Contingencies for additional information on the Company's leases. 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 be required to 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 instruments such as reinsurance recoverables, loans and held to maturity securities, 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 condensed consolidated financial statements. See Note 10, Investments and Cash for the Company's current accounting policy with respect to available-for-sale securities. Adoption of 2016-01 Amendments under this ASU apply to the Company's FG VIE liabilities which the Company has historically elected to measure through the consolidated statements of operations in "fair value gains (losses) on FG VIE" under the fair value option. For FG VIE liabilities with recourse, the portion of the change in fair value caused by changes in ISCR must now be separately presented in OCI as opposed to the consolidated statements of operations. Interest income and interest expense are derived from the trustee reports and also included in “fair value gains (losses) on FG VIEs”. The inception to date change in fair value of the FG VIE liabilities with recourse attributable to the ISCR (i.e., the Company's own credit risk) is calculated by holding all current period assumptions constant for each security and isolating the effect of the change in the Company’s CDS spread from the most recent date of consolidation to the current period. In general, if the Company’s CDS spread tightens more value will be assigned to the Company’s credit; however, if the Company’s CDS widens, less value is assigned to the Company’s credit. On adoption of ASU 2016-01, the Company reclassified approximately $33 million of loss, net of tax, from retained earnings to AOCI. This amount represents the portion of the fair value of the FG VIE liabilities with recourse that related to the change in the Company's own credit risk from the date of consolidation through January 1, 2018. The accounting and disclosure of the FG VIE liabilities without recourse are unchanged. Adoption of ASU 2016-01 Up until December 31, 2017, the change in fair value of the preferred stock investments and certain other equity investments was recorded in OCI. Effective January 1, 2018, in accordance with ASU 2016-01, the change in the fair value of these investments is recorded in other income in the condensed consolidated statements of operations. On adoption of ASU 2016-01, on January 1, 2018, the Company reclassified a loss of approximately $1 million, net of tax, from AOCI to retained earnings. In addition, in accordance with ASU 2016-01, the Company elected the new measurement alternative for equity securities that were accounted for under the cost method as of December 31, 2017 because they did not have a readily determinable fair value. Effective January 1, 2018, these equity securities will be accounted at cost less any impairment, plus or minus the change resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer.
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 First Quarter 2018, 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 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	Adoption of 2016-01 Amendments under this ASU apply to the Company's FG VIE liabilities which the Company has historically elected to measure through the consolidated statements of operations in "fair value gains (losses) on FG VIE" under the fair value option. For FG VIE liabilities with recourse, the portion of the change in fair value caused by changes in ISCR must now be separately presented in OCI as opposed to the consolidated statements of operations. Interest income and interest expense are derived from the trustee reports and also included in “fair value gains (losses) on FG VIEs”. The inception to date change in fair value of the FG VIE liabilities with recourse attributable to the ISCR (i.e., the Company's own credit risk) is calculated by holding all current period assumptions constant for each security and isolating the effect of the change in the Company’s CDS spread from the most recent date of consolidation to the current period. In general, if the Company’s CDS spread tightens more value will be assigned to the Company’s credit; however, if the Company’s CDS widens, less value is assigned to the Company’s credit. On adoption of ASU 2016-01, the Company reclassified approximately $33 million of loss, net of tax, from retained earnings to AOCI. This amount represents the portion of the fair value of the FG VIE liabilities with recourse that related to the change in the Company's own credit risk from the date of consolidation through January 1, 2018. The accounting and disclosure of the FG VIE liabilities without recourse are unchanged.