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Long-Term Debt - Narrative (Details) - USD ($) |  | 9 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2017 | Sep. 30, 2017 | Aug. 25, 2017 | Dec. 31, 2016
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |
Scheduled principal payments on debt in 2017 |  | $ 0 |  |
Scheduled principal payments on debt in 2018 |  | 0 |  |
Scheduled principal payments on debt in 2019 |  | 350,000,000 |  |
Scheduled principal payments on debt in 2020 |  | 0 |  |
Scheduled principal payments on debt in 2021 |  | 0 |  |
Scheduled principal payments on debt in 2022 |  | 0 |  |
Scheduled principal payments on debt after 2022 |  | 953,000,000 |  |
AFG |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |
Revolving credit line |  | 500,000,000 |  |
Amount borrowed under revolving credit facility |  | $ 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 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Effective interest rate, percentage |  | 9.419% |  | 9.0624%
Interest rate swaps | Fair Value Hedge | Designated as Hedging Instrument | LIBOR | AFG |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Basis spread on variable interest rate |  | 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 |  |  |  | $ 1,000,000
Long-term debt | Interest rate swaps | Fair Value Hedge | Designated as Hedging Instrument | AFG | Maximum |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Net derivatives, at fair value |  | $ (1,000,000) |  |
Senior Notes | 9-7/8% Senior Notes due June 2019 |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |
Stated interest rate, percentage |  | 9.875% |  |
Senior Notes | 9-7/8% Senior Notes due June 2019 | AFG |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |
Stated interest rate, percentage |  | 9.875% |  | 9.875%
Senior Notes | 4.50% Senior Notes due June 2047 | AFG |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |
Senior Notes, aggregate principal amount | $ 350,000,000 |  |  |
Stated interest rate, percentage | 4.50% | 4.50% |  |
Senior Notes, issuance price as a percentage of par value | 99.46% |  |  |
Senior Notes | 3.50% Senior Notes due August 2026 | AFG |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |
Stated interest rate, percentage |  | 3.50% |  |
Senior Notes | 6-3/8% Senior Notes due June 2042 | AFG |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |
Stated interest rate, percentage |  | 6.375% |  |
Senior Notes, amount of the original debt instrument that was redeemed | $ 230,000,000 |  |  |
Senior Notes | 5-3/4% Senior Notes due August 2042 | AFG |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |
Stated interest rate, percentage |  | 5.75% |  |
Senior Notes, amount of the original debt instrument that was redeemed |  |  | $ 125,000,000 |
Subordinated Debentures | 6-1/4% Subordinated Debentures due September 2054 | AFG |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |
Stated interest rate, percentage |  | 6.25% |  |
Subordinated Debentures | 6% Subordinated Debentures due November 2055 | AFG |  |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |
Stated interest rate, percentage |  | 6.00% |  |

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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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