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Long-Term Debt - Narrative (Details) - USD ($) |  | 1 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2016 | Jun. 30, 2016 | Sep. 30, 2016 | Aug. 31, 2016 | Dec. 31, 2015
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Scheduled principal payments on debt in 2016 | $ 18,000,000 |  | $ 18,000,000 |  |
Scheduled principal payments on debt in 2017 | 0 |  | 0 |  |
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 in 2021 | 0 |  | 0 |  |
Scheduled principal payments on debt after 2021 | 958,000,000 |  | 958,000,000 |  |
Principal | 1,326,000,000 |  | 1,326,000,000 |  | $ 1,020,000,000
AFG |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Revolving credit line term |  | 5 years |  |  |
Revolving credit line | 500,000,000 |  | 500,000,000 |  |
Amount borrowed under 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 |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Interest rate description for revolving credit facility |  |  | LIBOR plus 0.875% (currently 1.518%) |  |
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 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Effective interest rate, percentage | 8.9493% |  | 8.9493% |  | 8.611%
Interest rate swaps | Fair Value Hedge | Designated as Hedging Instrument | LIBOR | AFG |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Basis spread on variable interest rate | 8.099% |  | 8.099% |  |
Long-term debt | Interest rate swaps | Fair Value Hedge | Designated as Hedging Instrument | AFG |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Net derivatives, at fair value | $ 6,000,000 |  | $ 6,000,000 |  | $ 2,000,000
Senior Notes | AFG |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Principal | $ 1,008,000,000 |  | $ 1,008,000,000 |  | 708,000,000
Senior Notes | 9-7/8% Senior Notes due June 2019 |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Stated interest rate, percentage | 9.875% |  | 9.875% |  |
Senior Notes | 9-7/8% Senior Notes due June 2019 | AFG |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Principal | $ 350,000,000 |  | $ 350,000,000 |  | $ 350,000,000
Stated interest rate, percentage | 9.875% |  | 9.875% |  | 9.875%
Senior Notes | 3-1/2% Senior Notes due August 2026 | AFG |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Senior Notes, aggregate principal amount |  |  |  | $ 300,000,000 |
Senior Notes, issuance price as a percentage of par value |  |  |  | 99.608% |
Principal | $ 300,000,000 |  | $ 300,000,000 |  | $ 0
Stated interest rate, percentage | 3.50% |  | 3.50% | 3.50% |
Senior Notes | 6-3/8% Senior Notes due June 2042 | AFG |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Principal | $ 230,000,000 |  | $ 230,000,000 |  | 230,000,000
Stated interest rate, percentage | 6.375% |  | 6.375% |  |
Senior Notes | 5-3/4% Senior Notes due August 2042 | AFG |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Principal | $ 125,000,000 |  | $ 125,000,000 |  | 125,000,000
Stated interest rate, percentage | 5.75% |  | 5.75% |  |
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
Stated interest rate, percentage | 6.25% |  | 6.25% |  |
Subordinated Debt | 6% Subordinated Debentures due November 2055 | AFG |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Principal | $ 150,000,000 |  | $ 150,000,000 |  | 150,000,000
Stated interest rate, percentage | 6.00% |  | 6.00% |  |
Revolving Credit Facility | National Interstate |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Revolving credit line | $ 100,000,000 |  | $ 100,000,000 |  |
Principal | 18,000,000 |  | 18,000,000 |  | $ 12,000,000
Revolving Credit Facility | Line of Credit | National Interstate |  |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |
Principal | $ 6,000,000 |  | $ 6,000,000 |  |

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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 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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