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Long-Term Debt (Details Textual) - USD ($) |  | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2015 | Sep. 30, 2015 | Dec. 31, 2014
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Scheduled principal payments on debt in 2015 (are less than) | $ 1,000,000 | $ 1,000,000 |
Scheduled principal payments on debt in 2016 | 10,000,000 | 10,000,000 |
Scheduled principal payments on debt in 2017 | 12,000,000 | 12,000,000 |
Scheduled principal payments on debt in 2018 | 0 | 0 |
Scheduled principal payments on debt in 2019 | 350,000,000 | 350,000,000 |
Scheduled principal payments on debt in 2020 | 0 | 0 |
Scheduled principal payments on debt after 2020 | 508,000,000 | 508,000,000 |
AFG [Member] |  |  |
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 |
National Interstate [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Revolving credit line | 100,000,000 | $ 100,000,000 |
Loans payable under National Interstate bank credit facility | $ 12,000,000 | $ 12,000,000 |
Interest rate description for revolving credit facility |  | six-month LIBOR plus 0.875% (currently 1.28%) |
LIBOR [Member] | AFG [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Interest rate on revolving debt facility | 1.375% |  |
LIBOR [Member] | AFG [Member] | Minimum [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Interest rate on revolving debt facility | 1.00% |  |
LIBOR [Member] | AFG [Member] | Maximum [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Interest rate on revolving debt facility | 1.875% |  |
LIBOR [Member] | National Interstate [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Effective interest rate, percentage | 1.28% | 1.28% |
LIBOR [Member] | National Interstate [Member] | Maximum [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Interest rate on revolving debt facility | 0.875% |  |
Interest rate swaps [Member] | Fair Value Hedge [Member] | Designated as Hedging Instrument [Member] | AFG [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Effective interest rate, percentage | 8.4362% | 8.4362% |
Interest rate swaps [Member] | Fair Value Hedge [Member] | Designated as Hedging Instrument [Member] | LIBOR [Member] | AFG [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Basis spread on variable interest rate | 8.099% | 8.099% |
Long-term debt [Member] | Interest rate swaps [Member] | Fair Value Hedge [Member] | Designated as Hedging Instrument [Member] | AFG [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Net derivatives, at fair value | $ 7,000,000 | $ 7,000,000 |
Senior Notes [Member] | 9-7/8% Senior Notes due June 2019 [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Stated interest rate, percentage | 9.875% | 9.875% |
Senior Notes [Member] | 9-7/8% Senior Notes due June 2019 [Member] | AFG [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Senior Notes | $ 350,000,000 | $ 350,000,000 | 350,000,000
Stated interest rate, percentage | 9.875% | 9.875% |
Senior Notes [Member] | 6-3/8% Senior Notes due June 2042 [Member] | AFG [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Senior Notes | $ 230,000,000 | $ 230,000,000 | 230,000,000
Stated interest rate, percentage | 6.375% | 6.375% |
Senior Notes [Member] | 5-3/4% Senior Notes due August 2042 [Member] | AFG [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Senior Notes | $ 125,000,000 | $ 125,000,000 | 125,000,000
Stated interest rate, percentage | 5.75% | 5.75% |
Senior Notes [Member] | 7% Senior Notes due September 2050 [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Senior Notes | $ 132,000,000 | $ 132,000,000 |
Senior Notes [Member] | 7% Senior Notes due September 2050 [Member] | AFG [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Senior Notes | $ 0 | $ 0 | $ 132,000,000
Stated interest rate, percentage | 7.00% | 7.00% |
Subordinated Debt [Member] | 6-1/4% Subordinated Debentures due September 2054 [Member] | AFG [Member] |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |
Stated interest rate, percentage | 6.25% | 6.25% |

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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing after the sixth fiscal year following the latest fiscal year. + References No definition available. + Details
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- Definition Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the sixth fiscal year following the latest fiscal year ended. + References No definition available. + Details
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- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + 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 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=6451184&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 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=49121117&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=49121117&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=56946850&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=56946850&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=56946850&loc=d3e41620-113959 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986 Reference 5: 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=56946850&loc=SL5618551-113959 Reference 6: 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=56946850&loc=SL5579245-113959 + 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 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 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 SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 + 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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- Definition Including the current and noncurrent portions, carrying value as of the balance sheet date of loans from a bank with maturities initially due after one year or beyond the normal operating cycle if longer. + References Reference 1: 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 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(a)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 + Details
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- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 + Details
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