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Long-Term Debt - Narrative (Details) - USD ($) | 1 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |  |
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Dec. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 11, 2017 | Nov. 30, 2017 | Aug. 31, 2017 | Jun. 30, 2017 | Nov. 10, 2016 | Aug. 31, 2016 | Nov. 30, 2015 | Sep. 30, 2015 | Jun. 30, 2015
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Scheduled principal payments on debt in next year | $ 0 | $ 0 |  |  |  |  |  |  |  |  |  |  |
Scheduled principal payments on debt in year two | 0 | 0 |  |  |  |  |  |  |  |  |  |  |
Scheduled principal payments on debt in year three | 0 | 0 |  |  |  |  |  |  |  |  |  |  |
Scheduled principal payments on debt in year four | 0 | 0 |  |  |  |  |  |  |  |  |  |  |
Scheduled principal payments on debt in year five | 0 | 0 |  |  |  |  |  |  |  |  |  |  |
Scheduled principal payments on debt after year five | 1,320,000,000 | 1,320,000,000 |  |  |  |  |  |  |  |  |  |  |
Principal | 1,318,000,000 | 1,318,000,000 | $ 1,308,000,000 |  |  |  |  |  |  |  |  |  |
Cash interest payments on long-term debt |  | 85,000,000 | 75,000,000 | $ 75,000,000 |  |  |  |  |  |  |  |  |
AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Revolving credit line | 500,000,000 | 500,000,000 |  |  |  |  |  |  |  |  |  |  |
Amount borrowed under AFG revolving credit facility | 0 | $ 0 | 0 |  |  |  |  |  |  |  |  |  |
Interest rate description of fair value hedge activity |  | three-month LIBOR plus 8.099% |  |  |  |  |  |  |  |  |  |  |
Interest rate description for revolving credit facility |  | 1.00% to 1.875% (currently 1.375%) over LIBOR |  |  |  |  |  |  |  |  |  |  |
LIBOR | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on revolving debt facility |  | 1.375% |  |  |  |  |  |  |  |  |  |  |
LIBOR | AFG | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on revolving debt facility |  | 1.00% |  |  |  |  |  |  |  |  |  |  |
LIBOR | AFG | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on revolving debt facility |  | 1.875% |  |  |  |  |  |  |  |  |  |  |
Interest rate swaps | Fair Value Hedge | Designated as Hedging Instrument | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Cash interest received on long-term debt affiliated with interest rate swap |  | $ 2,000,000 | 3,000,000 | $ 2,000,000 |  |  |  |  |  |  |  |  |
Interest rate swaps | Fair Value Hedge | Designated as Hedging Instrument | LIBOR | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable interest rate |  |  |  |  | 8.099% |  |  |  |  |  |  |  |
Long-term debt | Interest rate swaps | Fair Value Hedge | Designated as Hedging Instrument | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Net derivatives, at fair value |  |  | 1,000,000 |  |  |  |  |  |  |  |  |  |
Senior Notes | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Principal | 1,018,000,000 | 1,018,000,000 | 1,008,000,000 |  |  |  |  |  |  |  |  |  |
Senior Notes | 4.50% Senior Notes due June 2047 | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes, aggregate principal amount |  |  |  |  |  | $ 240,000,000 |  | $ 350,000,000 |  |  |  |  |
Senior Notes, issuance price as a percentage of par value |  |  |  |  |  | 102.374% |  | 99.46% |  |  |  |  |
Principal | $ 590,000,000 | $ 590,000,000 | 0 |  |  |  |  |  |  |  |  |  |
Interest rate on debt instruments | 4.50% | 4.50% |  |  |  | 4.50% |  | 4.50% |  |  |  |  |
Senior Notes | 3.50% Senior Notes due August 2026 | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes, aggregate principal amount |  |  |  |  |  | $ 125,000,000 |  |  |  | $ 300,000,000 |  |  |
Senior Notes, issuance price as a percentage of par value |  |  |  |  |  | 99.698% |  |  |  | 99.608% |  |  |
Principal | $ 425,000,000 | $ 425,000,000 | 300,000,000 |  |  |  |  |  |  |  |  |  |
Interest rate on debt instruments | 3.50% | 3.50% |  |  |  | 3.50% |  |  |  | 3.50% |  |  |
Senior Notes | 9-7/8% Senior Notes due June 2019 |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on debt instruments | 9.875% | 9.875% |  |  |  |  |  |  |  |  |  |  | 9.875%
Senior Notes | 9-7/8% Senior Notes due June 2019 | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes, amount of the original debt instrument that was redeemed |  |  |  |  | $ 350,000,000 |  |  |  |  |  |  |  |
Senior Notes, amount of the original debt instrument that was redeemed plus a make-whole premium |  |  |  |  | $ 388,000,000 |  |  |  |  |  |  |  |
Principal | $ 0 | $ 0 | $ 350,000,000 |  |  |  |  |  |  |  |  |  |
Interest rate on debt instruments | 9.875% | 9.875% | 9.875% |  | 9.875% |  |  |  |  |  |  |  | 9.875%
Senior Notes | 9-7/8% Senior Notes due June 2019 | Interest rate swaps | Fair Value Hedge | Designated as Hedging Instrument | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Make-whole premium included in the loss on the retirement of debt | $ (1,000,000) |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes | 6-3/8% Senior Notes due June 2042 | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes, amount of the original debt instrument that was redeemed |  |  |  |  |  |  |  | $ 230,000,000 |  |  |  |  |
Principal | $ 0 | $ 0 | $ 230,000,000 |  |  |  |  |  |  |  |  |  |
Interest rate on debt instruments | 6.375% | 6.375% |  |  |  |  |  | 6.375% |  |  |  |  |
Senior Notes | 5-3/4% Senior Notes due August 2042 | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes, amount of the original debt instrument that was redeemed |  |  |  |  |  |  | $ 125,000,000 |  |  |  |  |  |
Principal | $ 0 | $ 0 | 125,000,000 |  |  |  |  |  |  |  |  |  |
Interest rate on debt instruments | 5.75% | 5.75% |  |  |  |  | 5.75% |  |  |  |  |  |
Senior Notes | 7% Senior Notes due September 2050 |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes, amount of the original debt instrument that was redeemed |  |  |  |  |  |  |  |  |  |  |  | $ 132,000,000 |
Senior Notes | 7% Senior Notes due September 2050 | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate on debt instruments |  |  |  |  |  |  |  |  |  |  |  | 7.00% |
Subordinated Debt | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Principal | $ 300,000,000 | $ 300,000,000 | 300,000,000 |  |  |  |  |  |  |  |  |  |
Subordinated Debt | 6-1/4% Subordinated Debentures due September 2054 | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Principal | $ 150,000,000 | $ 150,000,000 | 150,000,000 |  |  |  |  |  |  |  |  |  |
Interest rate on debt instruments | 6.25% | 6.25% |  |  |  |  |  |  |  |  |  |  |
Subordinated Debt | 6% Subordinated Debentures due November 2055 | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes, aggregate principal amount |  |  |  |  |  |  |  |  |  |  | $ 150,000,000 |  |
Principal | $ 150,000,000 | $ 150,000,000 | $ 150,000,000 |  |  |  |  |  |  |  |  |  |
Interest rate on debt instruments | 6.00% | 6.00% |  |  |  |  |  |  |  |  | 6.00% |  |
Revolving Credit Facility | National Interstate |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  |  | $ 18,000,000 |  |  |  |
Revolving Credit Facility | Line of Credit | National Interstate |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  |  | $ 6,000,000 |  |  |  |
Make-whole premium | Senior Notes | 9-7/8% Senior Notes due June 2019 | AFG |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Make-whole premium included in the loss on the retirement of debt | $ (38,000,000) |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Represents the debt instrument’s issuance price as a percentage of the debt instrument's face amount. + References No definition available. + Details
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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Fair value amount of debt instrument that was repurchased. + References No definition available. + Details
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- Definition Face (par) amount of the original debt instrument that was repurchased. + References No definition available. + Details
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- Definition The percentage points added to the reference rate to compute the variable rate on the interest rate derivative. + References No definition available. + Details
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- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 + Details
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- Definition Contains all pertinent information about an entity's fair value hedging activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579240-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41620-113959 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959 + Details
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- Definition Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 50 -Section 40 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12317-112629 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 50 -Section 40 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12355-112629 + Details
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- Definition Amount of cash paid for interest. Includes, but is not limited to, payment to settle zero-coupon bond attributable to accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4297-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585 + Details
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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Description of the interest rate for the amounts borrowed under the credit facility, including the terms and the method for determining the interest rate (for example, fixed or variable, LIBOR plus a percentage, increasing rate, timing of interest rate resets, remarketing provisions). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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