Accounting Policies (Policies)	12 Months Ended
Accounting Policies (Policies)	Dec. 31, 2014
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The consolidated financial statements include the accounts of American Financial Group, Inc. (“AFG”) and its subsidiaries. Certain reclassifications have been made to prior years to conform to the current year’s presentation. All significant intercompany balances and transactions have been eliminated. The results of operations of companies since their formation or acquisition are included in the consolidated financial statements. Events or transactions occurring subsequent to December 31, 2014, and prior to the filing of this Form 10-K, have been evaluated for potential recognition or disclosure herein. The preparation of the financial statements in conformity with U.S. generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Changes in circumstances could cause actual results to differ materially from those estimates.
Fair Value Measurements	Fair Value Measurements Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants on the measurement date. The standards establish a hierarchy of valuation techniques based on whether the assumptions that market participants would use in pricing the asset or liability (“inputs”) are observable or unobservable. Observable inputs reflect market data obtained from independent sources, while unobservable inputs reflect AFG’s assumptions about the assumptions market participants would use in pricing the asset or liability. Other than the recording of the acquisition of Summit Holding Southeast, Inc. and its related companies (see Note B — “Acquisitions and Sales of Businesses”), AFG did not have any significant nonrecurring fair value measurements of nonfinancial assets and liabilities in 2014 or 2013.
Investments	Investments Fixed maturity and equity securities classified as “available for sale” are reported at fair value with unrealized gains and losses included in accumulated other comprehensive income (“AOCI”) in AFG’s Balance Sheet. Fixed maturity and equity securities classified as “trading” are reported at fair value with changes in unrealized holding gains or losses during the period included in net investment income. Mortgage and policy loans are carried primarily at the aggregate unpaid balance. Premiums and discounts on fixed maturity securities are amortized using the interest method; mortgage-backed securities (“MBS”) are amortized over a period based on estimated future principal payments, including prepayments. Prepayment assumptions are reviewed periodically and adjusted to reflect actual prepayments and changes in expectations. Gains or losses on securities are determined on the specific identification basis. When a decline in the value of a specific investment is considered to be other-than-temporary at the balance sheet date, a provision for impairment is charged to earnings (included in realized gains (losses) on securities) and the cost basis of that investment is reduced. If management can assert that it does not intend to sell an impaired fixed maturity security and it is not more likely than not that it will have to sell the security before recovery of its amortized cost basis, then the other-than-temporary impairment is separated into two components: (i) the amount related to credit losses (recorded in earnings) and (ii) the amount related to all other factors (recorded in other comprehensive income). The credit-related portion of an other-than-temporary impairment is measured by comparing a security’s amortized cost to the present value of its current expected cash flows discounted at its effective yield prior to the impairment charge. Both components are shown in the Statement of Earnings. If management intends to sell an impaired security, or it is more likely than not that it will be required to sell the security before recovery, an impairment charge to earnings is recorded to reduce the amortized cost of that security to fair value.
Derivatives	Derivatives Derivatives included in AFG’s Balance Sheet are recorded at fair value. Changes in fair value of derivatives are included in earnings, unless the derivatives are designated as cash flow hedges. Derivatives that do not qualify for hedge accounting under GAAP consist primarily of (i) components of certain fixed maturity securities (primarily interest-only MBS) and (ii) the equity-based component of certain annuity products (included in annuity benefits accumulated) and related call options (included in other investments) designed to be consistent with the characteristics of the liabilities and used to mitigate the risk embedded in those annuity products. To qualify for hedge accounting, at the inception of a derivative contract, AFG formally documents the relationship between the terms of the hedge and the hedged items and its risk management objective. This documentation includes defining how hedge effectiveness and ineffectiveness will be measured on a retrospective and prospective basis. Changes in the fair value of derivatives that are designated and qualify as highly effective cash flow hedges are recorded in AOCI and are reclassified into earnings when the variability of the cash flows from the hedged items impact earnings. Any hedge ineffectiveness is immediately recorded in current period earnings. When the change in the fair value of a qualifying cash flow hedge is included in earnings, it is included in the same line item in the Consolidated Statement of Earnings as the cash flows from the hedged item. Qualifying highly effective cash flow hedges include interest rate swaps, which are used to mitigate interest rate risk related to certain floating-rate securities included in AFG’s portfolio of fixed maturity securities.
Goodwill	Goodwill Goodwill represents the excess of cost of subsidiaries over AFG’s equity in their underlying net assets. Goodwill is not amortized, but is subject to an impairment test at least annually. An entity is not required to complete the quantitative annual goodwill impairment test on a reporting unit if the entity elects to perform a qualitative analysis and determines that it is more likely than not that the reporting unit’s fair value exceeds its carrying amount.
Reinsurance	Reinsurance Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured policies. AFG’s property and casualty insurance subsidiaries report as assets (i) the estimated reinsurance recoverable on paid and unpaid losses, including an estimate for losses incurred but not reported, and (ii) amounts paid or due to reinsurers applicable to the unexpired terms of policies in force. Payable to reinsurers includes ceded premiums due to reinsurers as well as ceded premiums retained by AFG’s property and casualty insurance subsidiaries under contracts to fund ceded losses as they become due. AFG’s insurance subsidiaries also assume reinsurance from other companies. Earnings on reinsurance assumed is recognized based on information received from ceding companies. A subsidiary cedes life insurance policies to a third party on a funds withheld basis whereby the subsidiary retains the assets (securities) associated with the reinsurance contract. Interest is credited to the reinsurer based on the actual investment performance of the retained assets. This reinsurance contract is considered to contain an embedded derivative (that must be adjusted to fair value) because the yield on the payable is based on a specific block of the ceding company’s assets, rather than the overall creditworthiness of the ceding company. AFG determined that changes in the fair value of the underlying portfolio of fixed maturity securities is an appropriate measure of the value of the embedded derivative. The securities related to this contract are classified as “trading.” The adjustment to fair value on the embedded derivative offsets the investment income recorded on the adjustment to fair value of the related trading portfolio.
Deferred Policy Acquisition Costs (''DPAC'')	Deferred Policy Acquisition Costs (“DPAC”) Policy acquisition costs (principally commissions, premium taxes and certain underwriting and policy issuance costs) directly related to the successful acquisition or renewal of an insurance contract are deferred. DPAC also includes capitalized costs associated with sales inducements offered to fixed annuity policyholders such as enhanced interest rates and premium and persistency bonuses. For the property and casualty companies, DPAC is limited based upon recoverability without any consideration for anticipated investment income and is charged against income ratably over the terms of the related policies. A premium deficiency is recognized if the sum of expected claims costs, claims adjustment expenses and unamortized acquisition costs exceed the related unearned premiums. A premium deficiency is first recognized by charging any unamortized acquisition costs to expense to the extent required to eliminate the deficiency. If the premium deficiency is greater than unamortized acquisition costs, a liability is accrued for the excess deficiency and reported with unpaid losses and loss adjustment expenses. DPAC related to annuities is deferred to the extent deemed recoverable and amortized, with interest, in relation to the present value of actual and expected gross profits on the policies. Expected gross profits consist principally of estimated future investment margin (estimated future net investment income less interest credited on policyholder funds) and surrender, mortality, and other life and annuity policy charges, less death, annuitization and guaranteed withdrawal benefits in excess of account balances and estimated future policy administration expenses. To the extent that realized gains and losses result in adjustments to the amortization of DPAC related to annuities, such adjustments are reflected as components of realized gains (losses) on securities. DPAC related to traditional life and health insurance is amortized over the expected premium paying period of the related policies, in proportion to the ratio of annual premium revenues to total anticipated premium revenues. See “Life, Accident and Health Reserves” below for details on the impact of loss recognition on the accounting for traditional life and health insurance contracts. DPAC includes the present value of future profits on business in force of annuity and life, accident and health insurance companies acquired (“PVFP”). PVFP represents the portion of the costs to acquire companies that is allocated to the value of the right to receive future cash flows from insurance contracts existing at the date of acquisition. PVFP is amortized with interest in relation to expected gross profits of the acquired policies for annuities and universal life products and in relation to the premium paying period for traditional life and health insurance products. DPAC and certain other balance sheet amounts related to annuity, long-term care and life businesses are also adjusted, net of tax, for the change in expense that would have been recorded if the unrealized gains (losses) from securities had actually been realized. These adjustments are included in unrealized gains (losses) on marketable securities, a component of AOCI in AFG’s Balance Sheet.
Managed Investment Entities	Managed Investment Entities A company is considered the primary beneficiary of, and therefore must consolidate, a variable interest entity (“VIE”) based primarily on its ability to direct the activities of the VIE that most significantly impact that entity’s economic performance and the obligation to absorb losses of, or receive benefits from, the entity that could potentially be significant to the VIE. AFG manages, and has investments in, collateralized loan obligations (“CLOs”) that are VIEs (see Note H — “Managed Investment Entities”). Both the management fees (payment of which is subordinate to other obligations of the CLOs) and the investments in the CLOs are considered variable interests. AFG has determined that it is the primary beneficiary of the CLOs because (i) its role as asset manager gives it the power to direct the activities that most significantly impact the economic performance of the CLOs and (ii) it has exposure to CLO losses (through its investments in the CLO debt tranches) and the right to receive benefits (through its subordinated management fees and returns on its investments), both of which could potentially be significant to the CLOs. Because AFG has no right to use the CLO assets and no obligation to pay the CLO liabilities, the assets and liabilities of the CLOs are shown separately in AFG’s Balance Sheet (at fair value). AFG has elected the fair value option for reporting on the CLO assets and liabilities to improve the transparency of financial reporting related to the CLOs. The excess of fair value of the CLOs’ assets over the fair value of the liabilities is recorded in AFG’s Balance Sheet as appropriated retained earnings — managed investment entities, representing amounts that ultimately will inure to the CLO debt holders. The net gain or loss from accounting for the CLO assets and liabilities at fair value is separately presented in AFG’s Statement of Earnings. CLO earnings attributable to AFG’s shareholders represent the change in fair value of AFG’s investments in the CLOs (including distributions) and management fees earned. All other CLO earnings (losses) are not attributable to AFG’s shareholders and will ultimately inure to the CLO debt holders. As a result, such CLO earnings (losses) are included in net earnings (loss) attributable to noncontrolling interests in AFG’s Statement of Earnings and in appropriated retained earnings — managed investment entities in the Balance Sheet. As the CLOs approach maturity (2016 to 2026), it is expected that earnings (losses) attributable to noncontrolling interests will reduce appropriated retained earnings towards zero as the fair values of the assets and liabilities converge and the CLO assets are used to pay the CLO debt. At December 31, 2014, assets and liabilities of managed investment entities included $17 million in assets and $12 million in liabilities of a temporary warehousing entity that was established in connection with the formation of a new CLO that is expected to close in the second quarter of 2015. Upon closing, all warehoused assets are expected to be transferred to the new CLO and the liabilities will be repaid. In August 2014, the FASB issued Accounting Standards Update (“ASU”) 2014-13 to address the diversity in practice regarding the accounting for assets and liabilities of a consolidated collateralized financing entity (such as a CLO) when an election has been made to account for that entity’s assets and liabilities at fair value. As discussed above, the fair values of a CLO’s assets may differ from the fair values of its liabilities even though the liabilities only have recourse to the assets, which results in “appropriated retained earnings — managed investment entities” in AFG’s Balance Sheet. Under ASU 2014-13, AFG will have the option to set the carrying value of the CLO liabilities equal to the fair value of the CLO assets (which have more observable fair values) as an alternative to reporting those liabilities at fair value. Under this alternative, CLO earnings attributable to AFG’s shareholders would continue to be measured by the change in the fair value of AFG’s investments in the CLOs and management fees earned. However, as a result of setting the carrying value of the CLO liabilities equal to the fair value of the CLO assets, there would no longer be any excess carrying value of CLO assets over the carrying value of CLO liabilities to be reported as “appropriated retained earnings — managed investment entities” in AFG’s Balance Sheet or any CLO earnings to be attributed to noncontrolling interests in AFG’s Statement of Earnings. If AFG elects to continue to measure both the CLO assets and liabilities at fair value, ASU 2014-13 will require amounts currently reflected as “appropriated retained earnings — managed investment entities” to be reclassified to unappropriated retained earnings in the Balance Sheet and amounts currently attributed to noncontrolling interests in the Statement of Earnings to be included in earnings attributable to AFG shareholders. Although ASU 2014-13 is not required to be adopted until 2016, AFG intends to early adopt the guidance effective January 1, 2015 (as permitted) and to elect the alternative measurement guidance on a modified retrospective basis. As a result, the amount reported as “appropriated retained earnings — managed investment entities” in AFG’s Balance Sheet at December 31, 2014 will effectively be reclassified to “liabilities of managed investment entities” on January 1, 2015. While the ASU will have an impact on the future presentation of individual CLO-related line items in AFG’s Statement of Earnings in future periods, it will have no overall impact on AFG’s net earnings attributable to shareholders.
Unpaid Losses and Loss Adjustment Expenses	Unpaid Losses and Loss Adjustment Expenses The net liabilities stated for unpaid claims and for expenses of investigation and adjustment of unpaid claims represent management’s best estimate and are based upon (i) the accumulation of case estimates for losses reported prior to the close of the accounting period on direct business written; (ii) estimates received from ceding reinsurers and insurance pools and associations; (iii) estimates of unreported losses (including possible development on known claims) based on past experience; (iv) estimates based on experience of expenses for investigating and adjusting claims; and (v) the current state of the law and coverage litigation. Establishing reserves for asbestos, environmental and other mass tort claims involves considerably more judgment than other types of claims due to, among other things, inconsistent court decisions, an increase in bankruptcy filings as a result of asbestos-related liabilities, novel theories of coverage, and judicial interpretations that often expand theories of recovery and broaden the scope of coverage. Loss reserve liabilities are subject to the impact of changes in claim amounts and frequency and other factors. Changes in estimates of the liabilities for losses and loss adjustment expenses are reflected in the Statement of Earnings in the period in which determined. Despite the variability inherent in such estimates, management believes that the liabilities for unpaid losses and loss adjustment expenses are adequate.
Annuity Benefits Accumulated	Annuity Benefits Accumulated Annuity receipts and benefit payments are recorded as increases or decreases in annuity benefits accumulated rather than as revenue and expense. Increases in this liability for interest credited are charged to expense and decreases for policy charges are credited to other income. For certain products, annuity benefits accumulated also includes reserves for accrued persistency and premium bonuses, guaranteed withdrawals and excess benefits expected to be paid on future deaths and annuitizations (“EDAR”). The liabilities for EDAR and guaranteed withdrawals are accrued for and modified using assumptions consistent with those used in determining DPAC and DPAC amortization, except that amounts are determined in relation to the present value of total expected assessments. Total expected assessments consist principally of estimated future investment margin, surrender, mortality, and other life and annuity policy charges, and unearned revenues once they are recognized as income. Annuity benefits accumulated also includes amounts advanced from the Federal Home Loan Bank of Cincinnati.
Unearned Revenue	Unearned Revenue Certain upfront policy charges on annuities are deferred as unearned revenue (included in other liabilities) and recognized in net earnings using the same assumptions and estimated gross profits used to amortize DPAC.
Life, Accident and Health Reserves	Life, Accident and Health Reserves Liabilities for future policy benefits under traditional life, accident and health policies are computed using the net level premium method. Computations are based on the original projections of investment yields, mortality, morbidity and surrenders and include provisions for unfavorable deviations unless a loss recognition event (premium deficiency) occurs. Claim reserves and liabilities established for accident and health claims are modified as necessary to reflect actual experience and developing trends. For long-duration contracts (such as traditional life and long-term care policies), loss recognition occurs when, based on current expectations as of the measurement date, existing contract liabilities plus the present value of future premiums (including reasonably expected rate increases) are not expected to cover the present value of future claims payments and related settlement and maintenance costs (excluding overhead) as well as unamortized acquisition costs. If a block of business is determined to be in loss recognition, a charge is recorded in earnings in an amount equal to the excess of the present value of expected future claims costs and unamortized acquisition costs over existing reserves plus the present value of expected future premiums (with no provision for adverse deviation). The charge is recorded first to reduce unamortized acquisition costs and then as an additional reserve (if unamortized acquisition costs have been reduced to zero). In addition, reserves for traditional life and long-term care policies are subject to adjustment for loss recognition charges that would have been recorded if the unrealized gains from securities had actually been realized. This adjustment is included in unrealized gains (losses) on marketable securities, a component of AOCI in AFG’s Balance Sheet.
Variable Annuity Assets and Liabilities	Variable Annuity Assets and Liabilities Separate accounts related to variable annuities represent the fair value of deposits invested in underlying investment funds on which AFG earns a fee. Investment funds are selected and may be changed only by the policyholder, who retains all investment risk. AFG’s variable annuity contracts contain a guaranteed minimum death benefit (“GMDB”) to be paid if the policyholder dies before the annuity payout period commences. In periods of declining equity markets, the GMDB may exceed the value of the policyholder’s account. A GMDB liability is established for future excess death benefits using assumptions together with a range of reasonably possible scenarios for investment fund performance that are consistent with DPAC capitalization and amortization assumptions.
Premium Recognition	Premium Recognition Property and casualty premiums are earned generally over the terms of the policies on a pro rata basis. Unearned premiums represent that portion of premiums written which is applicable to the unexpired terms of policies in force. On reinsurance assumed from other insurance companies or written through various underwriting organizations, unearned premiums are based on information received from such companies and organizations. For traditional life, accident and health products, premiums are recognized as revenue when legally collectible from policyholders. For interest-sensitive life and universal life products, premiums are recorded in a policyholder account, which is reflected as a liability. Revenue is recognized as amounts are assessed against the policyholder account for mortality coverage and contract expenses.
Noncontrolling Interests	Noncontrolling Interests For Balance Sheet purposes, noncontrolling interests represents the interests of shareholders other than AFG in consolidated entities. In the Statement of Earnings, net earnings and losses attributable to noncontrolling interests represents such shareholders’ interest in the earnings and losses of those entities.
Income Taxes	Income Taxes Deferred income taxes are calculated using the liability method. Under this method, deferred income tax assets and liabilities are determined based on differences between financial reporting and tax bases and are measured using enacted tax rates. A valuation allowance is established to reduce total deferred tax assets to an amount that will more likely than not be realized. AFG recognizes the tax benefits of uncertain tax positions only when the position is more likely than not to be sustained under examination by the appropriate taxing authority. Interest and penalties on AFG’s reserve for uncertain tax positions are recognized as a component of tax expense.
Stock-Based Compensation	Stock-Based Compensation All share-based grants are recognized as compensation expense on a straight-line basis over their vesting periods based on their calculated fair value at the date of grant. AFG uses the Black-Scholes pricing model to measure the fair value of employee stock options. See Note K — “Shareholders’ Equity” for further information.
Benefit Plans	Benefit Plans AFG provides retirement benefits to qualified employees of participating companies through the AFG 401(k) Retirement and Savings Plan, a defined contribution plan. AFG makes all contributions to the retirement fund portion of the plan and matches a percentage of employee contributions to the savings fund. Company contributions are expensed in the year for which they are declared. AFG and many of its subsidiaries provide health care and life insurance benefits to eligible retirees. AFG also provides postemployment benefits to former or inactive employees (primarily those on disability) who were not deemed retired under other company plans. The projected future cost of providing these benefits is expensed over the period employees earn such benefits.
Earnings Per Share	Earnings Per Share Although basic earnings per share only considers shares of common stock outstanding during the period, the calculation of diluted earnings per share includes the following adjustments to weighted average common shares related to stock-based compensation plans: 2014 – 2.0 million, 2013 – 1.9 million and 2012 – 1.7 million. AFG’s weighted average diluted shares outstanding excludes the following anti-dilutive potential common shares related to stock compensation plans: 2014 – 1.0 million, 2013 – 1.1 million and 2012 – 1.8 million. Adjustments to net earnings attributable to shareholders in the calculation of diluted earnings per share were nominal in the 2014, 2013 and 2012 periods.
Statement of Cash Flows	Statement of Cash Flows For cash flow purposes, “investing activities” are defined as making and collecting loans and acquiring and disposing of debt or equity instruments and property and equipment. “Financing activities” include obtaining resources from owners and providing them with a return on their investments, borrowing money and repaying amounts borrowed. Annuity receipts, surrenders, benefits and withdrawals are also reflected as financing activities. All other activities are considered “operating.” Short-term investments having original maturities of three months or less when purchased are considered to be cash equivalents for purposes of the financial statements.